Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 10 5 2006 10.5194/hess-10-755-2006 http://www.hydrol-earth-syst-sci.net/10/755/2006/ http://www.hydrol-earth-syst-sci.net/10/755/2006/hess-10-755-2006.html http://www.hydrol-earth-syst-sci.net/10/755/2006/hess-10-755-2006.pdf 755 767 2006-10-17 Estimating spatial mean root-zone soil moisture from point-scale observations A. J. Teuling R. Uijlenhoet F. Hupet E. E. van Loon P. A. Troch Hydrology and Quantitative Water Management Group, Wageningen University, Wageningen, The Netherlands Department of Environmental Sciences and Land Use Planning, Université Catholique de Louvain, Louvain-la-Neuve, Belgium Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, The Netherlands Department of Hydrology and Water Resources, The University of Arizona, Tucson, Arizona, USA Root zone soil moisture is a key variable in many land surface hydrology models. Often, however, there is a mismatch in the spatial scales at which models simulate soil moisture and at which soil moisture is observed. This complicates model validation. The increased availability of detailed datasets on space-time variability of root-zone soil moisture allows for a posteriori analysis of the uncertainties in the relation between point-scale observations and the spatial mean. In this paper we analyze three comprehensive datasets from three different regions. We identify different strategies to select observation sites. For instance, sites can be located randomly or according to the rank stability concept. For each strategy, we present methods to quantify the uncertainty that is associated with this strategy. In general there is a large correspondence between the different datasets with respect to the relative uncertainties for the different strategies. For all datasets, the uncertainty can be strongly reduced if some information is available that relates soil moisture content at that site to the spatial mean. However this works best if the space-time dynamics of the soil moisture field are known. Selection of the site closest to the spatial mean on a single random date only leads to minor reduction of the uncertainty with respect to the spatial mean over seasonal timescales. Since soil moisture variability is the result of a complex interaction between soil, vegetation, and landscape characteristics, the soil moisture field will be correlated with some of these characteristics. Using available information, we show that the correlation with leaf area index or a wetness coefficient alone is insufficient to predict if a site is representative for the spatial mean soil moisture content. Al-Kaisi, M., Brun, L., and Enz, J.: Transpiration and evapotranspiration from maize as related to leaf area index, Agric. Forest Meteorol., 48, 111–116, 1989. Albertson, J. and Montaldo, N.: Temporal dynamics of soil moisture variability: 1. Theoretical basis, Water Resour. Res., 39, 1274, \doi10.1029/2002WR001616, 2003. Anderson, M. and Kneale, P.: Topography and hillslope soil water relationhips in a catchment of low relief, J. Hydrol., 47, 115–128, 1980. Baldocchi, D., Falge, E., Gu, L., Olson, R., Hollinger, D., Running, S., Anthoni, P., Bernhofer, C., Davis, K., Evans, R., Fuentes, J., Goldstein, A., Katul, G., Law, B., Lee, X., Malhi, Y., Meyers, T., Munger, W., Oechel, W., Paw, K., Pilegaard, K., Schmid, H., Valentini, R., Verma, S., Vesala, T., Wilson, K., and Wofsy, S.: FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities, Bull. Am. Meteorol. Soc., 82, 2415–2434, 2001. Bárdossy, A. and Lehmann, W.: Spatial distribution of soil moisture in a small catchment. Part 1: geostatistical analysis, J. Hydrol., 206, 1–15, 1998. Bell, K., Blanchard, B., Schmugge, T., and Witczak, M.: Analysis of surface moisture variations within large-field sites, Water Resour. Res., 16, 796–810, 1980. Chen, Y.: Letter to the Editor on "rank stability or temporal stability", Soil Sci. Soc. Am. J., 70, 306, \doi10.2136/sssaj2005.0290l, 2006. Comegna, V. and Basile, A.: Temporal stability of spatial patterns of soil water storage in a cultivated Vesuvian soil, Geoderma, 62, 299–310, 1994. Cosh, M., Stedinger, J., and Brutsaert, W.: Variability of surface soil moisture at the watershed scale, Water Resour. Res., 40, W12513, \doi10.1029/2004WR003487, 2004. Crave, A. and Gascuel-Odoux, C.: The influence of topography on time and space distribution of soil surface water content, Hydrol. Process., 11, 203–210, 1997. Famiglietti, J., Rudnicki, J., and Rodell, M.: Variability in surface moisture content along a hillslope transect: Rattlesnake Hill, Texas, J. Hydrol., 210, 259–281, 1998. Fitzjohn, C., Ternan, J., and Williams, A.: Soil moisture variability in a semi-arid gully catchment: implications for runoff and erosion control, Catena, 32, 55–70, 1998. Gómez-Plaza, A., Alvarez-Rogel, J., Albaladejo, J., and Castillo, V.: Spatial patterns and temporal stability of soil moisture across a range of scales in a semi-arid environment, Hydrol. Processes, 14, 1261–1277, 2000. Grant, L., Seyfried, M., and McNamara, J.: Spatial variation and temporal stability of soil water in a snow-dominated, mountain catchment, Hydrol. Processes, 18, 3493–3511, \doi10.1002/hyp.5798, 2004. Grayson, R. and Western, A.: Towards areal estimation of soil water content from point measurements: time and space stability of mean response, J. Hydrol., 207, 68–82, 1998. Grayson, R., Western, A., Chiew, F., and Blöschl, G.: Preferred states in spatial soil moisture patterns: Local and nonlocal controls, Water Resour. Res., 33, 2897–2908, 1997. Hawley, M., Jackson, T., and McCuen, R.: Surface soil moisture variation on small agricultural watersheds, J. Hydrol., 62, 179–200, 1983. Hupet, F. and Vanclooster, M.: Intraseasonal dynamics of soil moisture variability within a small agricultural maize cropped field, J. Hydrol., 261, 86–101, 2002. Hupet, F. and Vanclooster, M.: Sampling strategies to estimate field areal evapotranspiration fluxes with a soil water balance approach, J. Hydrol., 292, 262–280, \doi10.1016/j.jhydrol.2004.01.006, 2004. Hupet, F. and Vanclooster, M.: Micro-variability of hydrological processes at the maize row scale: implications for soil water content measurements and evapotranspiration estimates, J. Hydrol., 303, 247–270, \doi10.1016/j.jhydrol.2004.07.017, 2005. Hupet, F., Bogaert, P., and Vanclooster, M.: Quantifying the local-scale uncertainty of estimated actual evapotranspiration, Hydrol. Processes, 18, 3415–3434, \doi10.1002/hyp.1504, 2004. Jacobs, J., Mohanty, B., Hsu, E., and Miller, D.: SMEX02: Field scale variability, time stability and similarity of soil moisture, Remote Sens. Environ., 92, 436–446, \doi10.1016/j.rse.2004.02.017, 2004. Kachanoski, R. and de Jong, E.: Scale dependence and the temporal persistence of spatial patterns of soil water storage, Water Resour. Res., 24, 85–91, 1988. Loague, K.: Soil water content at R-5. Part 1. Spatial and temporal variability, J. Hydrol., 139, 233–251, 1992. Martínez-Fernández, J. and Ceballos, A.: Mean soil moisture estimation using temporal stability analysis, J. Hydrol., 312, 28–38, \doi10.1016/j.jhydrol.2005.02.007, 2005. Mohanty, B. and Skaggs, T.: Spatio-temporal evolution and time-stable characteristics of soil moisture within remote sensing footprints with varying soil, slope, and vegetation, Adv. Water Resour., 24, 1051–1067, 2001. Nyberg, L.: Spatial variability of soil water content in the covered catchment at Gårdsjön, Sweden, Hydrol. Processes, 10, 89–103, 1996. Pachepsky, Y., Guber, A., and Jacques, D.: Temporal persistence in vertical distributions of soil moisture contents, Soil Sci. Soc. Am. J., 69, 347–352, 2005. Petrone, R., Price, J., Carey, S., and Waddington, J.: Statistical characterization of the spatial variability of soil moisture in a cutover peatland, Hydrol. Processes, 18, 41–52, \doi10.1002/hyp.1309, 2004. Price, A. and Bauer, B.: Small-scale heterogeneity and soil-moisture variability in the unsaturated zone, J. Hydrol., 70, 277–293, 1984. Qiu, Y., Fu, B., Wang, J., and Chen, L.: Spatial variability of soil moisture content and its relation to environmental indices in a semi-arid gully catchment of the Loess Plateau, China, J. Arid Environ., 49, 723–750, 2001. Reynolds, S.: The gravimetric method of soil moisture determination, Part III, An examination of factors influencing soil moisture variability, J. Hydrol., 11, 288–300, 1970. Ryu, D. and Famiglietti, J.: Characterization of footprint-scale surface soil moisture variability using Gaussian and beta distribution functions during the Southern Great Plains 1997 SGP97 hydrology experiment, Water Resour. Res., 41, W12433, \doi10.1029/2004WR003835, 2005. Schenk, H. and Jackson, R.: The global biogeography of roots, Ecol. Monogr., 72, 311–328, 2002. Schume, H., Jost, G., and Katzensteiner, K.: Spatial-temporal analysis of the soil water content in a mixed Norway spruce (\mbox\itPicea abies (L.) Karst.) – European beech (\mbox\itFagus sylvatica L.) stand, Geoderma, 112, 273–287, 2003. Seyfried, M.: Spatial variability constraints to modeling soil water at different scales, Geoderma, 85, 231–254, 1998. Svetlitchnyi, A., Plotnitskiy, S., and Stepovaya, O.: Spatial distribution of soil moisture content within catchments and its modelling on the basis of topographic data, J. Hydrol., 277, 50–60, \doi10.1016/S0022-1694(03)00083-0, 2003. Teuling, A. and Troch, P.: Improved understanding of soil moisture variability dynamics, Geophys. Res. Lett., 32, L05404, \doi10.1029/2004GL021935, 2005. Vachaud, G., Passerat De Silans, A., Balabanis, P., and Vauclin, M.: Temporal stability of spatially measured soil water probability density function, Soil Sci. Soc. Am. J., 49, 822–828, 1985. Western, A. and Grayson, R.: The Tarrawarra data set: soil moisture patterns, soil characteristics, and hydrological flux measurements, Water Resour. Res., 34, 2765–2768, 1998. Western, A., Blöschl, G., and Grayson, R.: Geostatistical characterisation of soil moisture patterns in the Tarrawarra catchment, J. Hydrol., 205, 20–37, 1998. Western, A., Grayson, R., Blöschl, G., Willgoose, G., and McMahon, T.: Observed spatial organization of soil moisture and its relation to terrain indices, Water Resour. Res., 35, 797–810, 1999. Wilson, D., Western, A., and Grayson, R.: Identifying and quantifying sources of variability in temporal and spatial soil moisture observations, Water Resour. Res., 40, W02507, \mbox\doi10.1029/2003WR002306, 2004. Wilson, D., Western, A., and Grayson, R.: A terrain and data-based method for generating the spatial distribution of soil moisture, Adv. Water Resour., 28, 43–54, \doi10.1016/j.advwatres.2004.09.007, 2005. Wood, E.: Effects of soil moisture aggregation on surface evaporative fluxes, J. Hydrol., 190, 397–412, 1997.