Soil spatial variability and its consequences on simulated water balance

Scaling theory, based on the similar media concept, provides a basis for representing soil spatial variability in terms of a single stochastic variable, the scaling factor α, which is related to the microscopic characteristic length of the soil. Based on the measured properties of α (log normally distributed with an arithmetic mean of 1.0 and coefficient of variation of 0.6), the effects of soil spatial variability on the water balance components of a grassland watershed near Chickasha, Oklahoma, were studied by using a simulation model. Monthly simulations of evapotranspiration, surface runoff, deep drainage, and change in soil water storage for the year 1973 were computed for several values of α representing soils with widely ranging hydraulic properties. At any time, vegetation and meteorological parameters were the same for all the soils. Our studies reveal the complexities of soil-plant-atmospheric interactions in evaluating the influence of soil variability on water balance and underline the limitations of drawing generalizations. The conclusions are highly dependent on the extent of variability (coefficient of variation of α), the frequency distribution function of the scaling factor, and the soil-plant-weather combination.

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