Observed spatial organization of soil moisture and its relation to terrainindices

We analyze the degree of spatial organization of soil moisture and the abil ity of terrain attributes to predict that organization. By organization we mean systematic spatial variation or consistent spatial patterns. We use 13 observed spatial patterns of soil moisture, each based on over 500 point m easurements, from the 10.5 ha Tarrawarra experimental catchment in Australi a. The measured soil moisture patterns exhibit a high degree of organizatio n during wet periods owing to surface and subsurface lateral redistribution of water. During dry periods there is little spatial organization. The sha pe of the distribution function of soil moisture changes seasonally and is influenced by the presence of spatial organization. Generally, it is quite different from the shape of the distribution functions of various topograph ic indices. A correlation analysis found that In(a), where a is the specifi c upslope area, was the best univariate spatial predictor of soil moisture for wet conditions and that the potential radiation index was best during d ry periods. Combinations of In(a) or In(a/tan(beta)), where beta is the sur face slope, and the potential solar radiation index explain up to 61% of th e spatial variation of soil moisture during wet periods and up to 22% durin g dry periods. These combinations explained the majority of the topographic ally organized component of the spatial variability of soil moisture a post eriori. A scale analysis indicated that indices that represent terrain conv ergence (such as In(a) or In(a/tan(beta))) explain variability at all scale s from 10 m up to the catchment scale and indices that represent the aspect of different hillslopes (such as the potential solar radiation index) expl ain variability at scales from 80 m to the catchment scale. The implication s of these results are discussed in terms of the organizing processes and i n terms of the use of terrain attributes in hydrologic modeling and scale s tudies. A major limitation on the predictive power of terrain indices is th e degree of spatial organization present in the soil moisture pattern at th e time for which the prediction is made.