Modeling runoff and runon in a desert shrubland ecosystem, Jornada Basin, New Mexico

Abstract A new two-dimensional (2D) distributed parameter model is developed to simulate overland flow in two small, semiarid shrubland watersheds in the Jornada Basin, southern New Mexico. The model is event-based and represents the watershed by an array of 1-m 2 cells, in which the cell size is approximately equal to the average area of the shrubs. In the model, flow directions and volumes are computed by a second-order predictor–corrector finite difference scheme, which is employed to solve the 2D kinematic wave equation. Thus, flow routing is computed implicitly and may vary in response to flow conditions. The model uses only six parameters for which values are obtained from field surveys and rainfall simulation experiments. The model underpredicts runoff from the watersheds because the measured values of saturated hydraulic conductivity K s for intershrub areas are too high. The likely reason for this overestimation is that values of K s were obtained from runoff plot experiments conducted at the beginning of summer on surfaces with degraded seals, whereas most summer storms occur on surfaces that have experienced recent rainfall and have well-developed seals. Model performance is much improved when K s is treated as a calibration parameter. The importance of runon infiltration in supplying water to shrubs is investigated for a range of rainfall and antecedent soil moisture conditions. On average, runon infiltration accounts for between 3% and 20% of the total infiltration under a shrub. The most favorable conditions for runon infiltration are an initially wet soil and a low mean rainfall rate.