Estimating Soil Moisture Storage in the Root Zone From Surface Measurements

Numerical simulations of the dynamics of soil water in the root zone of a vegetated field show that the matric suction profile can eventually acquire the same shape as the relative root density profile. The suction at any depth is proportional to the email protected@ ing root density raised to a power that depends on soil texture. This behavior is found only for fully grown root systems when the soil water distribution is controlled primarily by the water absorption by the roots; this becomes more likely as the soil dries out. That this power law is a solution of the continuity equation will also be demonstrated analytically. This functional relationship is used to show that, under these restrictive conditions, root zone soil moisture storage is a linear function of volumetric moisture at a particular depth, with the slope changing with depth but fixed in time. This hypothesis is supported by both laboratory and field soil moisture profile measurements, which were taken in fields with fully grown plants. No other model parameters need to be estimated, and the good agreement found constitutes an independent validation of the soil water extraction model under the plant and soil conditions described. Potential applications of these results include estimating soil moisture storage in the root zone from remotely sensed surface soil moisture and estimating the root profile from matric potential profile measurements.