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

TitleModeling runoff in a desert shrubland ecosystem, Jornada Basin, New Mexico
Publication TypeThesis
Year of Publication1999
AuthorsHowes DA
Number of Pages204
Date Published1999
UniversitySUNY at Buffalo
CityBuffalo, NY
Thesis TypePh.D. Dissertationpp
Accession NumberJRN00280
Call Number00723
Keywordsdissertation, dissertations, hydrology, grassland, hydrology, rainfall simulation, hydrology, runon, hydrology, shrubland, hydrology, surface runoff, model, runoff, model, runon, runoff, model, theses, thesis, water flow, model
AbstractTwo new distributed parameter models, a one-dimensional (1D) model and a two-dimensional (2D) model, are developed to simulate overland flow in two small semiarid shrubland watersheds in the Jornada Basin, southern New Mexico. Both models are event-based and operate at the scale of the individual shrub, each watershed being represented as a set of 1-m2 cells. In the 1D model, the flow network is fixed for the duration of a rainfall event and is generated using the Tarboton (1997) routing algorithm. The volume of flow through this network is computed by numerically solving the one-dimensional kinematic wave equation using a first-order finite difference scheme. In the 2D model flow directions and volumes are computed by a second-order predictor-corrector finite difference scheme (Davis, 1988), which is employed to solve the two-dimensional kinematic wave equation. Thus, flow routing is computed implicitly and may vary in response to flow conditions. Each model uses only six parameters for which values are obtained from field surveys and rainfall simulation experiments. Both models underpredict runoff from the watersheds due to the measured values of saturated hydraulic conductivity Ks for intershrub areas being too high. This is because the runoff plot experiments used to estimate Ks were 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 better developed seals. Model performance is much improved when Ks 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 using the 2D model. On average, runon infiltration accounts for between 3 and 20% of the total infiltration in the soil under a shrub. The most favorable conditions for runon infiltration are an initially wet soil and a low mean rainfall rate. The two models developed in this study are fundamentally different, but the performances of the two models are remarkably similar. The principal difference between the models is that the 1D model performs better than the 2D model where flow is concentrated into well defined threads or rills. In contrast, the 2D model is better suited to simulating dispersed interrill flow on hillslopes with no rills or channels. Thus, there are advantages and disadvantages to both models, and the choice of model should be guided by the needs of the investigator.
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