POSTERS FOR COMPUTER SESSION

Computer Session

GIS-Assisted Modeling of Soil Erosion and Runoff in a Watershed

by E P Paningbatan and R L Lanuza¹

A GIS-aided methodology of modeling the spatial and temporal distribution of runoff and soil erosion in a landscape is currently being developed and validated in some sub-catchments of the Manupali watershed in Lantapan, Bukidnon, Philippines. This modeling exercise is envisioned to help study the impact of land use change and cropping systems on soil erosion, water quality and water quantity in a watershed. It is specifically designed to assist identify the erosion "hot spots" in a watershed and evaluate the effectiveness of soil conservation practices.

The inputs to run the model include time series rainfall rates, digital elevation map (DEM), soil map, land use map, cropping system map, and monitoring stations map. Slope map and local drain direction map are generated from the DEM. On the other hand, The outputs include time series maps of runoff, sediment concentration and soil erosion. Also, the water discharge, sediment concentration and amount of soil loss at the outlet monitoring stations are also predicted.

GIS-based PCRASTER that run on ordinary desktop computers which has the capability of supporting dynamic modeling is used in the simulation.

Runoff (R) in each raster cell is calculated from the difference of rainfall (P) and infiltration (I). Infiltration rate is generated as a function of soil, cropping system and land use. Surface water is routed to neighboring cells using a Local Drain Direction (LDD), a built-in subroutine of the GIS software. Water flow velocity is estimated using the Mannings equations.

Sediment concentration (C) is calculated using the simplified Rose equation;

C = 2700 S(1-Co)

       Where:             C = efficiency of sediment entertainment
                                 S = sine of slope angle

Co = fraction of surface area that is not exposed to erosion.

To measure discharge rates in the field, water height recorders and flow meters with data logger attachment are installed at the monitoring stations strategically located in the watershed. Rainfall rate and duration are measured using tipping bucket type pluviometer with data logging device. Likewise, sediment concentration of runoff is also measured at the monitoring stations.

Footnote_______________

¹ Department of Soil Science, University of the Philippines Los Banos, College, Laguna, Philippines