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SCALING OF SOIL MOISTURE A Hydrologic Perspective
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30 Mar 2002 8 15 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. 150 WESTERN GRAYSON BLO SCHL, Soil moisture is variable in space and time and many moisture dependent pro. cesses are nonlinear This leads to scale effects that need to be understood if we. are to make accurate predictions of the behavior of hydrologic systems where it. is generally necessary to aggregate in space and or time Similarly many other. spatial and temporal fields e g soil vegetation topography meteorology that. influence soil moisture and other hydrologic responses are variable The conse. quence of this is that scale effects are complex making hydrologic simulation and. prediction very challenging This review concentrates on the scale characteristics. and the scaling of soil moisture in the root zone focusing on catchment scale. spatial patterns with some reference to temporal patterns. For both measurements and models scale can be thought of as consisting. of a triplet of characteristics support spacing and extent Blo schl Sivapalan. 1995 Figure 1 illustrates each component of the scale triplet Support is the area. or time over which a measurement averages the underlying variations or over. which a model assumes homogenous conditions As support increases variability. decreases due to the effects of averaging and small scale features disappear Spac. ing is the separation between points at which measurements are made or between. computational points in a model As the spacing increases the amount of detail. resolved decreases leading to an apparent increase in the spatial size of features. Interestingly the variability apparent in the data is unaffected Western Blo schl. 1999 Extent refers to the total coverage of the measurements or model As extent. increases larger scale features are included in the data and both the variability. and the average size of the features tend to increase. The effect of changing each component of the scale triplet individually by a. factor of three is illustrated in Figure 2 Figure 2a shows a background of large. scale variation essentially a hill with a set of smaller scale features the letters. of the word scale superimposed Figure 2b shows the loss of small scale features. resulting from increasing the support by passing a three by three pixel averaging. window over the data Figure 2c shows the loss of detail in the small scale features. when a higher spacing is used Figure 2d shows the loss of large scale features. when the extent is limited Clearly scale has an effect on the characteristics of the. information obtained from a data set or model, Figure 1 The scale triplet after Blo schl Sivapalan 1995. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. SCALING OF SOIL MOISTURE 151, We often need to use information from one scale at another scale This is the. scaling problem Beven 1995 and it is challenging because practical problems. typically involve using information from one scale to make predictions at a scale. that has a greater information requirement i e taking sparse data and estimating. intervening values converting Figure 2c to 2a taking areal averages and disaggre. gating them converting Figure 2b to 2a or taking results at small extents and ex. trapolating to larger areas converting Figure 2d to 2a Also scaling often involves. changing more than one component of the scale triplet at a time e g often both sup. port and spacing change together The essence of successful scaling is to distill the. key patterns from information at one scale and to use these to make good predictions. at another scale Such efforts often rely on a suite of relevant supplementary infor. mation ranging from digital elevation models to meteorological time series data. It is important to note at this point that there are several uses of the term scaling. in soil science and hydrology We have chosen the broad definition outlined above. and have opted to confine most of our discussion to soil moisture however a brief. overview of related concepts is in order In soil science scaling is sometimes lim. ited to concepts of similar media in the context of scaling soil hydraulic properties. and flow and transport equations Kut lek Nielsen 1994 Sposito 1998a The. essential aim has been to either describe soil heterogeneity or the integrated effect. of soil heterogeneity on flow or transport processes The concept of geometri. cally similar media was first introduced by Miller Miller 1955a b to relate soil. hydraulic properties to the pore structure Using capillary theory this assumption. leads to the soil water pressure head functions and hydraulic conductivity functions. being related to a microscopic length scale that can be used to characterize het. erogeneity Further research has led to approaches for scaling static soil hydraulic. behavior Warrick 1990 Attempts are also being made to scale the dynamic unsat. urated flow equations directly Haverkamp et al 1998 Sposito 1998b however. limitations related to the dependence on the type of boundary condition used in the. analyses exist at present Stochastic approaches are also being used to understand. the effects of heterogeneity on flow and transport in porous media Jury et al 1987. Russo 1998 and to derive effective hydraulic properties for analysis of flow and. transport processes in heterogeneous soils at field scales Yeh 1998 Research into. the characteristics and representation of soil heterogeneity can contribute to the. broader understanding of the effect of scale on the hydrologic cycle to the extent. that soil heterogeneity is one of several scale dependent influences on hydrologic. fluxes and states In summary these approaches relate soil hydraulic properties. to media properties whereas in this review the focus is on inferring the spatial. distribution of soil moisture from a few point data and on inferring the temporal. dynamics of soil moisture from a few snapshots, In hydrology the term scaling is sometimes used to refer to techniques based. on fractal approaches or scaling invariance Gupta Waymire 1998 Rinaldo. Rodr guez Iturbe 1998 Rodr guez Iturbe Rinaldo 1997 These have been used. to analyze properties of stream networks and topography However in this paper. we use the term scaling exclusively to denote a transfer of information from one. scale to another as discussed above, 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR.
152 WESTERN GRAYSON BLO SCHL, The literature contains a number of examples of important scale effects as. sociated with soil moisture variation Merz Plate 1997 studied the effect of. spatially variable patterns of soil moisture and soil infiltration characteristics on. the event rainfall runoff response at Weiherbach in southwest Germany using a. process based distributed rainfall runoff model Three types of spatial patterns. were considered uniform structured and random The structured patterns were. characterized by relatively wet gullies and dry hilltops and or by lower infiltration. capacity in the gullies Substantial differences in both peak discharge and event. runoff volume were found between each of the three types of patterns with initial. soil moisture being more important than the infiltration properties The differ. ences were dependent on the rainfall characteristics with medium sized events. influenced most by the spatial variability Similar studies have been conducted by. Grayson et al 1995 and Western et al 2001, Variability in the land surface can also have a significant impact on atmo. spheric processes Weaver Avissar 2001 studied the impact of variations in the. surface latent and sensible heat fluxes on mesoscale atmospheric circulation over. Oklahoma and Kansas using the Colorado State University Regional Atmospheric. Modeling System At the land surface boundary they applied a 2 km grid pattern of. land surface sensible and latent heat flux over a 252 km square area derived from. surface measurements The spatial variations were primarily due to vegetation pat. terns and moisture availability and they had characteristic scales of 20 100 km. The simulations suggested that strong mesoscale atmospheric circulations devel. oped under a range of synoptic conditions as a result of surface heterogeneity. They led to moisture being transported higher into the atmosphere and would be. expected to lead to increased cloud formation and precipitation The atmospheric. simulations were also compared with satellite cloud images which could not be. simulated realistically when ignoring the spatial variability of surface latent and. sensible heat fluxes, While the studies described above are model based and the conclusions there. fore depend on the validity of the model assumptions significant efforts have been. made to test the models using appropriate spatial data The studies indicate signifi. cant scale effects associated with variability in soil moisture or in the second case. evapotranspiration which is partially moisture controlled The effects on fluxes are. of practical significance in both cases but the characteristic scales of importance. are very different illustrating the problem specific nature of scaling issues. The rest of this paper is structured as follows First the key physical processes. controlling soil moisture are briefly discussed Second measurement of soil mois. ture by ground based and remote sensing techniques is discussed and the key scale. characteristics of different measurements are identified In the final section scaling. techniques applicable to soil moisture are presented including a brief discussion. of temporal scaling and then concentrating on spatial controls on soil moisture. scaling based on behavioral analysis and scaling based on process analysis. There are several published reviews that also provide useful information of. relevance to soil moisture scaling They include reviews on scaling in hydrology. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. SCALING OF SOIL MOISTURE 153, in general Blo schl Sivapalan 1995 Dunne et al 1975 Koltermann Gorelick. 1996 on land atmosphere interactions Entekhabi 1995 Giorgi Avissar 1997. Pielke 2001 and on soil moisture modeling Mahmood 1996 Sposito 1998a. presents a collection of papers on scaling in hydrology and soil science. SOIL MOISTURE PROCESSES, In this section we briefly describe the parts of the hydrologic cycle that directly.
affect soil moisture as well as some elementary soil physics More detailed treat. ments can be found in text books Dingman 1994 Hillel 1998 Kut lek Nielsen. 1994 Soil is a complex porous media containing mineral particles water and air. together with small amounts of organic matter and the soil biota Soil water is. a reservoir in the hydrologic cycle that influences runoff evapotranspiration and. drainage to groundwater systems and to streams There are two standard quantita. tive definitions of soil moisture gravimetric g and volumetric v g is defined. as the mass of water divided by the mass of dry soil and v is defined as the. volume of water divided by the volume of soil It is possible to convert between. the two using the dry soil bulk density b v b g Soil moisture is an integral. quantity that represents the average conditions in a finite volume of soil For proper. interpretation it is important to know the volume and particularly the depth range. that a soil moisture value represents, From a theoretical perspective the range in soil moisture is bounded by zero. and the soil porosity is the volume of soil pores divided by the volume of. soil In the field the practical lower limit on soil moisture is positive due to the. inability of plants to extract water below a particular level commonly called the. wilting point The bounded nature of soil moisture has important implications for. its statistical characteristics in space and time as is explored in more detail later. Figure 3a illustrates a standard one dimensional conceptualization of the soil. profile and the fluxes that influence the soil moisture stored in the profile Gen. erally the exchanges between the atmosphere and the soil dominate changes in. soil moisture The soil moisture store is primarily replenished by infiltration and. depleted by soil evaporation and plant transpiration The relative importance of. evaporation and transpiration depends on the vegetation cover with transpiration. dominating in well vegetated landscapes Fluxes between the soil and groundwater. or deeper parts of the regolith can be important in some contexts Drainage from. the soil profile is the primary source of recharge for many groundwater systems. and capillary rise from shallow groundwater tables can be an important source of. water replenishing the soil water store during drier periods. Included in Figure 3a is a series of soil moisture profiles measured for a clay. loam soil in Victoria Australia Both the amount of soil moisture and its dynamics. change with depth In the upper 50 cm soil moisture is strongly influenced by. the fluxes between the active root zone and the atmosphere the moisture here. is more variable than the moisture at depth Surface soil moisture also responds. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. 154 WESTERN GRAYSON BLO SCHL, Figure 3 a One dimensional conceptualization of fluxes affecting soil moisture This is. applicable where lateral flows are insignificant Also shown are the surface energy fluxes. The soil moisture depth profiles are from the Tarrawarra catchment Australia b A two. dimensional conceptualization of fluxes affecting soil moisture. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. SCALING OF SOIL MOISTURE 155, more quickly and so has both short and long timescale variability whereas the. moisture at depth is less responsive to short term variations in the fluxes across the. soil atmosphere interface, Figure 3b illustrates a standard conceptualization of a hillslope The key dif. ference between Figure 3a and 3b is that lateral flows now act to redistribute soil. water and influence runoff processes For significant lateral drainage to occur the. following conditions are necessary, Topographic relief surface slopes greater than a few percent.
An impeding layer in the soil profile limiting vertical drainage or anisotropy. between vertical and horizontal hydraulic conductivities Zavlasky Sinai. Sufficiently high moisture contents for periods long enough for flow to occur. over significant distances, In higher parts of the landscape lateral drainage can deplete the soil moisture. store This lateral drainage collects in convergent parts of the hillslope the hollows. and replenishes the soil moisture store in those areas often leading to soil saturation. and generation of saturation excess runoff It is important to emphasize that the. existence of lateral flow depends on the above conditions which only occur in. some landscapes and at some times, Figure 4 shows the changes in hydraulic conductivity and soil water tension. or soil suction with soil moisture content for a typical loam soil There is a rapid. decline in hydraulic conductivity or increase in resistance to flow as the soil dries. Also as smaller and smaller pores are emptied of water the tension increases due. to capillary effects The combined effect of these influences results in hydraulic. gradients dominated by gravity and relatively conductive soils at high moisture. contents Provided a soil is free to drain wet soils drain to the point where gravity. can no longer remove water from the soil pores The soil moisture at this point is the. field capacity f At lower moisture contents the hydraulic gradients are dominated. by soil water tension and the resistance to flow increases rapidly Because of the. Figure 4 Typical hydraulic conductivity and water retention curves. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. 156 WESTERN GRAYSON BLO SCHL, rapidly decreasing flow velocities as soil dries the spatial scale at which soil. water flow processes operate is very much smaller for dry soils than for wet soils. The changes in soil water tension also affect evapotranspiration by increasing the. energy required to extract water from the soil as soil moisture decreases This leads. to a marked reduction in evapotranspiration under dry conditions Eventually the. tension becomes so large that plants can no longer extract water from the soil. and they wilt This soil moisture content is referred to as the wilting point w. The plant available water paw is the soil water storage capacity that is readily. available to plants Paw f w D where D is the depth of soil exploited by. the plant roots, It is important to realize that typical soil hydraulic conductivity curves such as. that shown in Figure 4a often neglect the effects of plant roots and other soil biota. in creating macropores that can transmit water rapidly through the soil profile. effectively bypassing the soil matrix under certain conditions Beven Germann. 1982 Soil cracks can also be important in conveying water rapidly in both the. vertical and horizontal directions Western Grayson 2000. The change in evapotranspiration with moisture content has important implica. tions for both the moisture and energy fluxes involved in the interaction between. the atmosphere and the land surface The surface energy balance components are. shown in Figure 3a in addition to the moisture fluxes The energy and moisture. fluxes are intimately linked together via the evapotranspiration process When soil. moisture is evapotranspired a phase change from liquid water to water vapor oc. curs The moisture flux E and latent heat flux are linked by the latent heat of. vaporization of water x such that xE Soil moisture availability in conjunction. with atmospheric conditions controls the evapotranspiration and the partitioning. of incoming solar energy into latent and sensible heat fluxes Because of the ef. fects on atmospheric heating and the atmospheric moisture content land surface. interaction and the role of soil moisture in this interaction have become an area of. great interest to atmospheric modelers Improving our understanding in this area. has motivated and will continue to motivate a series of major interdisciplinary. studies many of which are coordinated under GEWEX Global Water and En. ergy Cycle Experiment see http www gewex com Scaling of soil moisture is. especially important for these studies because much of our understanding of soil. moisture and related processes is based on point and small catchment studies but. atmospheric modeling requires land surface predictions over large 5 500 km grid. sizes and global extents,MEASUREMENT OF SOIL MOISTURE.
Soil moisture is a difficult quantity to measure in a comprehensive manner There. are essentially two groups of approaches to moisture measurement remote sens. ing and ground based measurements The data collected using each of these ap. proaches have quite different scale characteristics. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. SCALING OF SOIL MOISTURE 157,Remote Sensing Measurement. Most remote sensing of soil moisture has concentrated on using microwave wave. lengths Dobson Ulaby 1998 Engman 2000 Jackson et al 1996 Both active. e g synthetic aperture radar SAR where a signal is sent from the sensor and the. returned signal measured and passive e g microwave radiometers that measure. the naturally emitted microwave radiation have been used These instruments re. spond to soil moisture in the top few mm to few cm of the soil profile depending. on the exact wavelength used so long as the vegetation canopy is not too dense Du. et al 2000 Compared with instruments using visible and infrared wavelengths. microwave instruments have the advantage of seeing through cloudy conditions. Engman 2000 From a user s perspective the most fundamental difference be. tween these instruments is their spatial and temporal resolution As implemented. at the present time on satellites or spaceborne SAR provides data with pixel sizes. of roughly 10 m although some aggregation is required to overcome the problem. of noise due to speckle This increases the effective pixel size at which sensible. interpretations of soil moisture can be made to roughly 100 m A W Western. T Sadek R B Grayson H N Turral in review Spaceborne SAR imagery has a. typical repeat time of about two weeks In contrast to SAR spaceborne microwave. radiometer instruments have spatial resolutions of tens of km that is three orders of. magnitude coarser than SAR but coverage is repeated daily Airborne microwave. radiometers including ESTAR electronically scanned thinned array radiometer. and PBMR push broom microwave radiometer have been used quite extensively. to map soil moisture at pixel sizes of hundreds of m during large scale field experi. ments such as FIFE MONSOON90 Washita 92 Jackson et al 1996 and SGP97. Jackson et al 1999, Several important practical issues for operational measurement of soil moisture. exist These are primarily related to the fact that the microwave signal responds to. the dielectric constant largely determined by soil moisture soil surface rough. ness and the vegetation canopy Dobson Ulaby 1998 Jackson et al 1996 This. means that several pieces of information are required to extract an estimate of soil. moisture from the signal Either this extra information can be preexisting mapping. or it can be remotely sensed however because the vegetation and soil roughness. are dynamic characteristics at least some generally need to be remotely sensed. Another important limitation of microwave remote sensing is that the instru. ments respond only to near surface soil moisture not to the soil water stores of. most interest to hydrologists which are the root zone and the whole profile One. approach to overcome this limitation is to assimilate a series of surface soil mois. ture measurements into a soil moisture model thereby estimating the moisture. profile Entekhabi et al 1994 Walker et al 2001 although the accuracy will be. highly dependent on the validity of the models, Another option is to use thermal remote sensing to measure land surface tem. perature and to infer evapotranspiration rates via a surface energy balance model. e g Bastiaanssen et al 1998 Because the evapotranspiration response depends. to some extent on root zone moisture availability this can provide semiquantitative. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. 158 WESTERN GRAYSON BLO SCHL, information on soil moisture availability however this approach can only be ap. plied during cloud free conditions and it is unlikely to provide the same degree. of quantification of absolute soil moisture content as microwave techniques. Ground Based Measurement, Ground based soil moisture measurements will provide more precise soil moisture.
data than remote sensing in the foreseeable future provided appropriate calibra. tion of instruments is performed It also allows measurement of soil moisture. over depths of more interest to hydrologists than does remote sensing Standard. techniques include thermogravimetric determination neutron scattering and mea. surement of dielectric properties of the soil One example of the latter method is. time domain reflectometry TDR Modern sensor and logging technology allow. routine monitoring at points and the Global Positioning System allows accurate. location of roving instruments such as the University of Melbourne s Terrain Data. Acquisition System TDAS Tyndale Biscoe et al 1998 Western Grayson. 1998 Highly detailed temporal and spatial soil moisture data sets such as those. from Tarrawarra Western Grayson 1998 and MARVEX Woods et al 2001. can be collected with these systems but logistical considerations mean it is only. possible to study small areas generally 1 km2, All of the common ground based soil moisture measurement techniques have. very small supports typically from 1 cm to 10 cm This is three to six orders of. magnitude smaller than remote sensing measurements and it enables collection. of more highly detailed spatial information than does remote sensing However. spacing between permanently installed moisture sensors varies widely from meters. up to hundreds of km depending on experimental objectives This means that. unlike remotely sensed soil moisture patterns moisture contents are measured in. a very small proportion of a study area Another key difference between ground. based techniques and remote sensing is that ground based sensors can be logged. providing detailed temporal patterns at a point whereas remote sensing typically. provides spatial patterns at points in time, There is a substantial amount of spatial variability in soil moisture Given the. small support and large spacing of ground based techniques interpretation of the. data is challenging Two possibilities exist The first is to make a large number. of measurements with sufficiently high spatial resolution to define the spatial. variability or the spatial pattern Western Grayson 1998 The second is to relate. the point scale measurements to areal soil moisture a promising possibility that. has been studied by Grayson Western 1998 Kachanoski de Jong 1988 and. Vachaud et al 1985 To be practical such an approach requires that a predictable. time stable relationship exist between point soil moisture and the spatial mean. Remote sensing and ground based measurement methods are in many ways. complementary Remote sensing provides excellent spatial coverage over large. areas but the shallow measurement depth confounding influence of vegetation. and surface roughness for SAR indirect nature of the method and relatively. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. SCALING OF SOIL MOISTURE 159, infrequent repeat cycles make use of the data problematic On the other hand. ground based methods can be applied over any depth accurately calibrated and. logged at any time scale but are essentially point measurements making spatial. interpretation difficult It is obvious that future work should focus on combining. these two data sources to exploit their complementary features. SCALING OF SOIL MOISTURE, In any problem it is important to be clear on the objectives of the scaling exercise. Where spatially or temporally variable quantities are important there is a range. of characteristics that can characterize the variation One possible classification is. given below approximately in order of increasing detail Some example applica. tions of each piece of information are then given in the context of spatial soil mois. ture It should be noted that all but the first of these is scale dependent How far we. need to progress down this list for any given problem will depend on our objectives. Central tendency i e mean median etc,Spread i e variance interquartile range etc.
Extremes i e percentage above or below a threshold a high or low per. Probability density function pdf, Spatial or temporal relationships correlation functions or variograms and. cross correlations or covariograms with say terrain connectivity see be. The actual pattern, The most fundamental problem in hydrology is to understand the water balance. Making measurements of the fluxes in and out of a soil profile is possible with. varying degrees of accuracy To close the water balance the change in the amount. of soil water and hence the mean soil moisture at the start and end of a period is. required If this change in soil moisture is to be measured to some specified accu. racy knowledge of the variance is also required to calculate the required number of. sampling points In the absence of spatial correlation between measurements this. can be done using standard statistical approaches based on Student s t distribution. An important runoff process in many landscapes is saturation excess runoff. which is runoff generated from areas of saturated soil To predict this runoff we need. to know about the wet extreme of the soil moisture distribution specifically we want. to know about the percentage of area with soil moisture above a threshold value. close to porosity It is likely that one would also need to know the mean moisture. and possibly the variance if one needed to predict rather than measure this area. Evapotranspiration processes depend on soil moisture in a nonlinear manner and. this can lead to important scale effects in atmospheric models Giorgi Avissar. 1997 Pielke 2001 One way in which the variability of soil moisture could be. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. 160 WESTERN GRAYSON BLO SCHL, accounted for in this situation is to integrate the evapotranspiration processes over. the area using the probability density function of the soil moisture as input in the. manner suggested by Beven 1995 This would provide the mean flux and the. spatial variability in the flux both of which may be important for atmospheric. There are a variety of reasons for which one may wish to know about spatial. relationships relevant to soil moisture that are discussed in more detail below One. example is for predicting the change in subgrid variance due to changing the grid. size in a model that parameterizes the effects of subgrid variability in soil moisture. using the variance If the correlation function is known the technique of regular. ization discussed later can then be employed to predict this change in variance. The most complete description of the variation in soil moisture in space is to. know the actual spatial pattern If the actual pattern is known then all the pre. ceding characteristics can be estimated from the data It is rare to know the actual. pattern of soil moisture from readily interpretable measurements Actual patterns. of soil moisture can provide initial conditions for or spatial data for testing of. distributed hydrologic models although most often such patterns are estimated. through some scaling analysis Recently actual patterns have been used to test. distributed models and they have been found to be extremely valuable in assess. ing model performance Grayson Blo schl 2000 particularly for soil moisture. Western Grayson 2000,TEMPORAL SCALING, Here we discuss briefly the temporal scale characteristics of soil moisture before. concentrating on the spatial scaling of soil moisture Figure 5 shows time series. of 0 30 cm and 30 60 cm volumetric soil moisture recorded at one point in the. Figure 5 Volumetric soil moisture variation at a mid slope location at Satellite Station. in the Mahurangi River catchment New Zealand The rapidly varying moisture is for. the top 30 cm layer and the slowly varying moisture is for the 30 60 cm layer. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. SCALING OF SOIL MOISTURE 161, Mahurangi River Catchment New Zealand Woods et al 2001 The largest scale.
feature of the time series is the seasonal variation in soil moisture This occurs in. response to seasonal changes in the balance between potential evapotranspiration. PET and precipitation as a consequence of seasonal changes in PET in this case. Superimposed on this seasonal cycle is a series of wetting and drying events with. time scales related to storm duration and inter storm period respectively The rate. of depletion during drying periods is mainly related to the rate of evapotranspiration. plus drainage divided by the rooting depth The contrast in the rates of change for. increasing and decreasing soil moisture are primarily related to the different flux. magnitudes for precipitation and evapotranspiration processes. Grayson et al 1997 discuss the presence of preferred states in the temporal. distribution of soil moisture Where PET dominates over precipitation soil mois. ture tends to be consistently low Similarly where precipitation dominates over. PET soil moisture tends to be consistently high This behavior is a consequence of. the bounded nature of soil moisture In many landscapes there is a seasonal shift. between these two states In landscapes where there is significant lateral move. ment of water this temporal behavior corresponds with a change in controls on the. spatial soil moisture pattern from being dominated by local vertical fluxes. Figure 3a during the dry state to being dominated by lateral fluxes Figure 3b. during the wet state, Several theoretical studies of soil moisture at a point have been conducted. using statistical dynamic models Initially these examined the average behavior. Eagleson 1978a b and later temporal dynamics was incorporated Milly 1994. 2001 Porporato et al 2001 Rodr guez Iturbe 1991a b Rodr guez Iturbe et al. 1999 Slightly different assumptions have been made by the various authors but. similar patterns of behavior have emerged from the analyses including bimodal. temporal pdfs of soil moisture under some circumstances D Odorico et al 2000. Milly 2001 which can be interpreted as preferred states These arise due to the. interaction between the random rainfall and the bounding of the soil moisture. in the model All of these models currently ignore seasonality in the forcing by. rainfall and evapotranspiration whereas the key feature leading to the preferred. states discussed by Grayson et al 1997 was the seasonal change in the balance. between PET and precipitation rates If these models incorporated seasonality in the. forcing we would expect that this would reinforce the bimodal behavior predicted. by the models The stochastic approach is clarifying the key characteristics of. the temporal dynamics of soil moisture within the hydrologic cycle and is also. providing insights into interactions between atmosphere and land surface e g. D Odorico et al 2000 Rodr guez Iturbe 1991a and also between the soil water. hydrology and ecological largely vegetation patterns Eagleson 1982a b Laio. et al 2001a b Rodr guez Iturbe et al 2001, Temporal correlation scales have been analyzed by Entin et al 2000 Vinnikov. Robock 1996 and Vinnikov Yeserkepova 1991 who divided the timescales. into short event and long components and found typical correlation scales of. about two months for the long component Entin et al 2000 found that temporal. scales were slightly less for the top 10 cm layer compared to the top 1 m layer. 30 Mar 2002 8 17 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. 162 WESTERN GRAYSON BLO SCHL, and that timescales increased with latitude Timescales also increased in winter. These latitudinal and seasonal effects were attributed to changes in PET These. studies lend support to the hypothesis that the temporal correlation scale of soil. moisture is linked to the ratio paw PET Delworth Manabe 1988 The corre. lation timescales for soil moisture are much longer than for precipitation due to. the memory of the soil moisture store which smoothes out the relatively rapid. variations in precipitation Also recycling of precipitation at continental scales. allows the possibility of feedback between the soil moisture store and atmosphere. and such recycling is thought to prolong soil moisture and precipitation anomalies. Entekhabi 1995,SPATIAL SCALING, In this section we first discuss the controls on spatial soil moisture patterns and then. examine techniques for the spatial scaling of soil moisture The scaling techniques. are divided into behavioral techniques and process based techniques Behavioral. techniques aim to quantify the apparent behavior of soil moisture patterns as a. function of scale and to use this quantification to predict the effects of chang. ing scale These techniques rely on data and statistical analysis which may be. combined with a conceptual understanding of process controls on soil moisture in. some cases In contrast process based techniques aim for a deeper understanding. of the physical processes causing the spatial patterns of soil moisture They use. this process understanding usually within a deterministic reductionist framework. of distributed water balance modeling to predict the effects of changing scale. Spatial Control of Soil Moisture, Figure 6 illustrates a number of spatial soil moisture patterns in which the ef.
fects of different spatial controls on soil moisture are evident At small scales. soil moisture responds to variations in vegetation Qui et al 2001 soil properties. Famiglietti et al 1998 topographically driven variations in lateral flow e g. Dunne Black 1970a b radiation e g Western et al 1999a and precipita. tion Figures 6a and 6b illustrate soil moisture patterns at Tarrawarra Western. Grayson 1998 where terrain plays an important role in lateral flow during. wet conditions Figure 6a but not during dry conditions Figure 6b and ra. diation is also important The soil moisture state has an important influence in. determining the controls on spatial soil moisture patterns here Grayson et al. 1997 This contrast in dominant processes can be used to advantage when cali. brating different process parameters in models Albertson Kiely 2001 The. primary effect of vegetation is on evapotranspiration from the soil profile how. ever canopy storage affects precipitation reaching the soil surface particularly in. forests Vertessy et al 2001 Vegetation can also influence infiltration properties. at the plant scale Seyfried Wilcox 1995 Soil properties can affect infiltration. during intense rainfall events they determine the saturated moisture content and. 20 Mar 2002 7 47 AR AR154 06 tex AR154 06 SGM LaTeX2e 2001 05 10 P1 GSR. SCALING OF SOIL MOISTURE 163, in conjunction with the vegetation the wilting point They also influence drainage. both vertical and lateral Figure 6c shows a soil moisture pattern on sandy soils at. Point Nepean Australia where consistently high soil moisture is associated with. areas of finer soils in the center and on the east side of the sampling area. As the spatial scale increases different sources of variation become apparent. Variation in vegetation shifts from the plant to the patch to the community scale. Some vegetation communities are associated with changes in moisture availability. for example riparian vegetation Soil properties vary as different soil types and. geologies are encountered Figures 6d and 6e show soil texture and a soil moisture. pattern respectively measured on June 18 1997 using ESTAR during SGP97. Jackson et al 1999 The correlation between soil texture and surface soil moisture. is clearly evident Variations in rainfall can occur at spatial scales as small as. hundreds of meters due to the passage of storm cells Goodrich et al 1995. however the long term effect on soil moisture variability may be at a larger space. scale as the soil store integrates precipitation over time and thus smoothes some. of the spatial and temporal variation in instantaneous rainfall rates Certainly at. spatial scales of kms examples of soil moisture variability due to spatial variability. in event rainfall depth can be found Figure 6f shows the effect of rainfall pattern. on soil moisture on June 30 1997 during the SGP97 field experiment Jackson. et al 1999, At still larger scales climatic variations in precipitation lead to substantial. changes in general soil moisture conditions between climate regions Variation in. humidity temperature and radiation also affect soil moisture through the evapo. transpiration process Figures 6g and 6h show the pattern of maximum vegetation. greenness a measure of vigor of vegetation growth across Australia during 1996. and 2000 mapped using data from AVHRR advanced very high resolution ra. diometer Climatic impacts are clear with inland Australia generally having low. amounts of vegetation due to low precipitation and soil moisture The interannual. variation in vegetation is also clear This is a response to variation in precipitation. and moisture availability between 1996 and 2000 1996 was a dry year in cen. tral Australia and wet in the Southeast and Southwest whereas 2000 was a wet. year with significant rainfall over large areas of inland Australia but drier in the. Southeast and Southwest, All of the factors affecting the distribution of soil moisture mentioned above are. correlated in space to some degree For example rainfall is likely to be more similar. for two points 1 m apart than for two points 1 km apart These spatial correlations. introduce spatial correlations into the soil moisture pattern In addition lateral. redistribution of soil water enhances spatial variation and correlation. Behavioral Scaling Techniques, In this section we review results relating to the statistical distribution of spatial. soil moisture spatial correlation of soil moisture and relationships between soil. moisture and surrogate variables The joint use of the latter two for spatial inter. polation is discussed at the end of the section All of these characterize different.

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COMPUTING SEC 09 SYLLABUS - University of Malta

COMPUTING SEC 09 SYLLABUS University of Malta

Technology. Structure of the syllabus The syllabus is organised into five main parts, as follows: 1. Computer Applications 2. Computer Architecture and Data Representation 3. Computer Systems 4. Algorithmic Problem Solving and Programming 5. Information and Communications Technology in Society Sequence of the syllabus does not necessarily dictate the order in which topics are to be taught ...

Outcomes of sustained low efficiency dialysis versus ...

Outcomes of sustained low efficiency dialysis versus

Hill, ON) and CA210 or Xenium 210 dialyzers (Baxter, Deerfield, IL). SLED sessions were targeted to 8 h in dur-ation (minimum 6 h) with blood and dialysate flows of

Design of Cantilever Sheet Pile Walls - January 2015

Design of Cantilever Sheet Pile Walls January 2015

design methods for sheet pile walls using the Truline 800 Series composite wall sections. Reinforcement Options Three options for the placement of reinforcement in the composite wall sections are typically used. These options are shown in Figures 1 to 3. Also shown in these figures are the dimensions bw, d, and h that describe the geometry of the reinforcement placement. Tension Side d = 3.73 ...

Reading First - U.S. Department of Education

Reading First U S Department of Education

Verbal Task on all words for Kindergarten an 1st Grade (Study 2) 0 10 20 30 40 50 60 70 80 90 100 Pre Kdg. Post Kdg. Pre 1st Post 1st Verbal Picture. Students provided with additional. review/practice/play more than. doubled. their rate of retention. of word meanings!! Beck et al. Does Direct Vocabulary Instruction Work?

Frequently Asked Questions - stagecrueltyfree.peta.org

Frequently Asked Questions stagecrueltyfree peta org

What Types of Companies Are on the "Don't Test" List? The list includes companies that make cosmetics, personal-care products, household-cleaning products, and other common household products. In the U.S., no law requires that these types of products be tested on animals, and companies can choose not to sell their products in countries such as China, where tests on animals are required for ...

Emergency Response to Terrorism Self-Study

Emergency Response to Terrorism Self Study

EMERGENCY RESPONSE TO TERRORISM SELF-STUDY . EMERGENCY RESPONSE TO TERRORISM: SELF-STUDY FEDERAL EMERGENCY MANAGEMENT AGENCY UNITED STATES FIRE ADMINISTRATION NATIONAL FIRE ACADEMY FOREWORD The Federal Emergency Management Agency (FEMA) was est ablished in 1979. FEMA's mission is to focus Federal effort on preparedness for, mitigation of, response to, and recovery from emergencies encompassing ...

Natural and Artificial Methods of Heat Resource ...

Natural and Artificial Methods of Heat Resource

for the regeneration of heat in rock masses are described. Keywords: Borehole heat exchanger, geoenergetics, heat pump, underground thermal energy storage keyword. 1. Introduction . Borehole thermal energy storages (BTESs) are increasing in interest and implementation, especially with systems for heating and/or cooling. The use of traditional fossil fuels only to keep buildings at . OPEN ...

Texas Tech University - ERIC

Texas Tech University ERIC

TESL-EJ 14.4, March 2011 Gorsuch Page 3 formulaic phrases and sentences) without false starts, undue hesitations, and violations of clause boundaries depends greatly ...

J RUDD S BOOK OF OSCE WISDOM - topendexam.com

J RUDD S BOOK OF OSCE WISDOM topendexam com

Paed 0.15mg/kg IV/IM (0.3 IN) Repeat at 5 mins 2nd Line Levetiracetam 40mg/kg (Max 3g - likely adult dose) NB: this is my own estimation not taken from source (off licence) 3rd Line (Decision to RSI is already made - simultaneous management) Phenytoin 20mg/kg IV . Jo Rudds Book of OSCE Wisdom. Transcribed August 2019. Andrew Smith 2019:2 OSCE Prep. 3rd Line (Ongoing seizure or altered ...

English as a Second Language in Relation with Verbal ...

English as a Second Language in Relation with Verbal

English as a Second Language in Relation with Verbal Fluency in SBK Women University Quetta Nadeem Malik Lecturer ... Koponen & Riggenbach (2000, p.6) ...