OBJECTIVES To develop hillslope and watershed erosion models for the Manupali subwatersheds based on the WEPP model; To simulate surface runoff, soil erosion and sediment yield for the selected watersheds; To test the applicability of the WEPP model for determining the effects of varying cover management practices on soil erosion and sediment yield in the selected watersheds.

The WEPP Model a process-based, distributed parameter, continuous computer simulation model designed for predicting water-induced soil erosion either on a hillslope or watershed scale based on the fundamental principles of stochastic weather generation, infiltration theory, hydrology, soil physics, plant science, hydraulics and mechanics of soil erosion and sediment transport

The WEPP Model (Cont’d) developed by the National Soil Erosion Research Laboratory (NSERL) of USDA's Agricultural Research Service Started in 1985 by NSERL;first model was released in 1989; DOS version was released in 1995;Windows-based version was first released in 2001 has proven applicability in all continents except Antarctica. In the Asian continent, it has already been applied in China but not yet in Southeast Asia much less the Philippines.

The WEPP Model Components

Climate Component Makes use of either measured or synthetic climatic data A rainfall disaggregation model using a double exponential function is also incorporated to generate time-rainfall breakpoint data from daily values The climatological parameters including disaggregated rainfall are used in estimating peak rate, duration and total amount of runoff

Hydrology Component Includes simulation of infiltration, surface runoff, soil evaporation, plant transpiration, percolation and subsurface flow Excess rainfall is consequently routed downslope by the WEPP model to estimate the overland flow hydrograph using a kinematic wave approach Peak runoff is calculated using either a semi- analytical solution of the kinematic wave equation or an approximate solution based on soil, rainfall, slope and friction factors

Hydrology Component (Cont’d.) The kinematic wave model for runoff routing Where: h = flow depth; q= unit discharge; x = distance; t = time

Plant Growth Component Simulates the plant and residue status above and below the soil surface Crop growth model is based on the EPIC model, which assumes phenological crop development based on daily accumulated heat units and has a harvest index for partitioning grain yield Canopy cover and height, biomass above and below ground surface, leaf area index, root growth and basal area are estimated on a daily basis

Soil Component Simulates the temporal variability of the various soil properties that influence the soil erosion process Maintains a daily accounting of the status of the soil and surface variables e.g. bulk density, saturated hydraulic conductivity, rill and interrill erodibility, critical shear stress, etc. Accounts for the effect of tillage, consolidation and rainfall on soil and surface variables

Management Component Accounts for the effect of the various land management practices on hydrology and erosion for a given site Determines the changes in soil physical properties, surface roughness and cover conditions due to tillage, crop harvest, grazing and other management activities

Erosion Component Fundamentally based on the concept that soil erosion is a process of detachment and transport Simulates both hillslope and channel erosion

Erosion Component (cont’d) Sediment continuity equation for hillslope model where: G = sediment load (kg/s/m); x = distance downslope (m); D f = rill erosion rate (kg/s/m 2 ); D i = interrill erosion rate (kg/s/m 2 )

Erosion Component (cont’d) Rill detachment when the hydraulic shear stress > critical shear stress of the soil and when sediment load < the sediment transport capacity where: Dc = detachment capacity by rill flow and Tc = sediment transport capacity in the rill

Erosion Component (cont’d) Detachment Capacity when the hydraulic shear stress > the critical shear where: Kr = rill erodibility parameter, τ f = flow shear stress acting on the soil; τ c = critical shear stress of the soil

Erosion Component (cont’d) Net Deposition when the sediment load > the sediment transport capacity where: Vf = effective fall velocity for the sediment, q = unit discharge, β = raindrop-induced turbulence coefficient

Erosion Component (cont’d.) Interrill sediment delivery = f( R.F. i 2, interrill erodibility, factors representing ground and canopy cover effects) Flow shear stress = f(average slope/gradient, Darcy-Weisbach friction factor) Sediment transport capacity is calculated based on modified Yalin sediment transport equation

General Methodology Secondary data collection Field visitation and primary data collection Test for adequacy and reliability of data Climatological data processing for BPCDG Breakpoint climate data generation using BPCDG WEPP hillslope model development WEPP watershed model development Simulation of hillslope and watershed soil erosion and sediment yield Analysis of simulation results

The Manupali Watershed The Philippines

Alanib Automated Weather Station

GPS unit used for coordinates verification

Bulogan Automated Weather Station

Kulasihan Automated Weather Station

Kulasihan Hydrologic Gaging Station

Tugasan Hydrologic Gaging Station

Demo of Sediment Discharge Sampling by TWG

Demo of Streamflow Measurement by TWG

Canal dimension measurement for Q

Sample WEPP Hillslope Erosion Model

Sample WEPP Hillslope Erosion Model after model execution

The WEPP Watershed Erosion Model for Maagnao watershed

The WEPP Watershed Erosion Model for Maagnao watershed during model execution

The WEPP Watershed Erosion Model for Maagnao watershed after model execution

WEPP Hillslope Model Simulation Results

Watershed Erosion Prediction Under Varying Land Cover Conditions

CONCLUSIONS The WEPP hillslope models for cropped areas at the test watersheds generated soil loss values comparable to measured values by previous researchers; The WEPP watershed model appear to overpredict surface runoff and sediment yield although better comparison could have been achieved had instantaneous hydrograph and sediment discharge data been available

CONCLUSIONS (cont’d.) The WEPP model proved to be applicable for simulating the effects of land cover conditions on sediment yield Increasing the fraction of watershed area cultivated and cropped results to increase in sediment yield based on WEPP simulation The applicability of the current version of the WEPP model in Philippine watersheds appears to be constrained by the unavailability and inadequacy of enormous input data

Results of this study could serve as framework for further model development in the light of more accurate and updated data and consequently as a tool for natural resource management and as basis for sound policy formulation on issues concerning land cover management in upland watersheds in the Philippines CONCLUSIONS (cont’d.)

ACKNOWLEDGEMENTS This study was funded by the USAID through SANREM- CRSP Environmental Research Grant Special thanks to the NSERL, USDA-ARS for the WEPP Model Thanks to -Heifer International Philippines for some secondary data -Dr. Bill Deutsch and the TWG for the streamflow and sediment discharge data - Dr. Vel Suminguit for his help during field visitation and additional data -Everyone in the SANREM family who contributed directly and indirectly

Thanks!!! Contact Address: Dr. Victor B. Ella Land and Water Resources Division Institute of Agricultural Engineering College of Engineering and Agro-Industrial Technology University of the Philippines Los Baños College, Laguna 4031 PHILIPPINES Tel/Fax: Tel/Fax: (63-49) Mobile Phone: Mobile Phone: or