Estimating profile water storage from surface zone soil moisture measurements under bare field conditions

A linear regression approach was used to estimate profile water storage from the storage in the surface zone. For a given thickness of the surface zone the correlations between the surface zone and profile water storages decreased as the profile depth increased; for a given profile depth, these correlations increased as the surface zone thickness increased. In general, the correlations were higher for irrigated corn fields than for bare fields. Results show that the profile depth for which water storage can be predicted from surface zone soil moisture data depends on the thickness of the surface zone, the cultural condition of the field, and the coefficient of determination considered adequate. An alternative procedure to estimate water storage in a deep profile under bare field conditions is based on the assumption that change in profile water storage over a period of time equals net surface flux over the same time. Surface fluxes are computed from surface zone soil moisture data and hydrologic properties of the soil. In a simulation study, net surface fluxes very nearly equaled changes in water storage of a 2.25-m-deep profile. For a field-measured data set, computed surface fluxes and those measured by a weighing lysimeter showed good overall agreement.