The Sibalom River Watershed Biophysical features and the application of the RUSLE soil erosion model Roel Burgers, MSc. student Dr. Gertjan (Ian) Geerling
Sibalom watershed Sulu Sea
Sibalom watershed Sulu Sea
Soil erosion Soil erosion depletes soil productivity for future generations. Soil erosion increases the sediment carried by rivers, and can increase its lateral movement. Soil erosion blocks downstream irrigation channels. Soil erosion increases sediment input in coastal areas, causing siltation that affects for example coral reefs. ….... (more?)
Soil erosion types Splash erosion Sheet erosion Gully erosion (non recoverable) Rill erosion
Erosion Example: sheet, rill and gully erosion (short movie)
Lateral erosion Durok Pang pang San Pedro Sibalom Topomap of 1950s In RED the 2001 rivercourse 1 kilometer
Erosion model A model is a simplified set of calculations, to mimic a real world process. • Measuring real erosion is sometimes difficult. • A model can cover a large area. • Future scenarios can be incorporated into the model.
Simple erosion model Sheet and rill erosion are included only
How the model works R * K * LS * C * P = Estimated erosion R Rainfall erosivity factor K Soil erodibility factor LS Slope length and angle C The cover and management factor P Support practice factor
Rain The rainfall erosivity factor is calculated based on the monthly rainfall of Valderama. Rain gauge used by NIA, Antique.
Soil data The characteristics of various Philippine soils were taken from David (1988) and the UNDP 1972 soil map of Panay. Soil erodibility is estimated from pH, Organic matter content (OM), Clay ratio (C), Sand (S) and Silt (Si) contents. K = [(0,043)*(pH)+ 0.62/OM + 0.0082*S - 0.0062*C ] * Si
Slope lenght The slope length is the amount of eroding slope before deposition starts. The longer the slope, the more erosion.
LU types Overview Forest Rice field Rice terrace Bare land Grassland Bushland Inland water)
Landuse Values for C factor influences on erosion
Prevention Well maintained rice terraces reduce sheet and rill erosion. Therefore on LU rice terrace the P factor changes according to the slope. Steeper slopes have lesser effect. If no rice terraces are present the P factor is 1.
Putting it all together Estimated erosion = R * K * LS * C * P So if we multiply all the points on the maps, the end result will be a map with estimated erosion for each point. The mean annual soil loss averaged over the whole area is about 2000 t ha-1 yr-1. Because we have no actual measurement of the erosion, we compare scenarios relatively.
Partly reforested • According to Philippine law the following areas should be reforested: • Uplands above 1 kilometer elevation • Slopes steeper than 18 degrees
Comparing the scenarios Relative sheet and rill erosion (partly reforested = 100)
Conclusions • The model canbeappliedusing free availablemaps and satellite images. • Rainfallmay have more effect, nowonly 1 value over the wholearea (Valderama). • Reforestationfollowingexistingpoliciescanreducesoilerosionby a factor 2.5 in the Sibalomriverwatershed.
Recommendations for LGUs and Agencies • Keep on measuring rain (NIA). Spread measurement locations over the whole watershed, also at locations higher in the mountains (>100m, >500m, >1 km). • Measure runoff and install water gauges in Sibalom river.(Also important for flood early warning systems!) • LGUs and barangays must co-operate within a watershed to create and implement a vision and management plan. • Need to review CLUP to accommodate watershed requirements • Integrate CLUPs at the provincial level to mainstream watershed management • Closer cooperation between LGUs and agencies, share available information, build one information databank for each watershed.
Recommendations for communities • Plant trees! Follow guidelines of DENR! • Not only the soil erodes, but also fertilizer and pesticides put onto it! These can have adverse effects downstream! • Effect of dead wood in rivers: leave (some) dead wood because it reduces erosion and increases habitat availability for fish and other aquatic species. • Initiate sloping agricultural land technology (terraces)
Thank you! • NIA • DENR • Municipalities San Jose, Sibalom, San Remigio • Process