The Sibalom River Watershed Biophysical features and the

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

Sibalom watershed Sulu Sea

Sibalom watershed Key facts Sulu Sea Area 66500 Ha Highest point in watershed 1584 Length Sibalom river to source 70 km Municipalities (also partly) 7 / 13 (including Iloilo side) Barangays (also partly) 135

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 Rill erosion Sheet erosion Gully erosion (non recoverable)

Erosion Example: sheet, rill and gully erosion (short movie)

Lateral (sideways) erosion San Remigio

Durok Lateral erosion Pang pang San Pedro Sibalom Topomap of 1950 s In RED the 2001 river course 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 Rain Land use Soil type Slope Soil Erosion Prevention Sheet and rill erosion are included only

How the model works R * K * LS * C * P = Estimated erosion R K LS C P Rainfall erosivity factor Soil erodibility factor Slope length and angle The cover and management factor Support practice factor

Rain The rainfall erosivity factor is calculated based on the monthly rainfall of Valderama. Year 1956 Annual 4750 Year 1964 Annual 3486 Year 1972 Annual 3151 Year 1980 1957 3839 1965 3600 1973 2734 1981 1958 2594 1966 3325 1974 3479 1982 1959 2918 1967 4513 1975 2945 1983 1960 1961 1962 3553 3452 3097 1968 1969 1970 3657 3958 3576 1977 1978 1984 1985 1986 1963 2920 1971 4501 1979 3356 3617 Incomplet e Annual Incomplet e 1946 3152 Incomplet e 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 p. H, Organic matter content (OM), Clay ratio (C), Sand (S) and Silt (Si) contents. K = [(0, 043)*(p. H)+ 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 1. 2. 3. 4. 5. 6. 7. 8. Overview Forest Rice field Rice terrace Bare land Grassland Bushland Inland water)

Landuse Values for C factor influences on erosion Forest Rice field Rice terrace Bare land Grassland Bush land Inland water 0. 002 0. 15 1 0. 007 0. 006 0. 15

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. Slope (%) 1 -2 3 -8 9 -12 13 -16 17 -20 21 -25 >25 P value 0. 12 0. 10 0. 12 0. 14 0. 16 0. 18 0. 20

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 (partly reforested = 100) Relative sheet and rill erosion 300 250 200 150 100 50 0 Present situation Partly reforested Everything forest

Conclusions • The model can be applied using free available maps and satellite images. • Rainfall may have more effect, now only 1 value over the whole area (Valderama). • Reforestation following existing policies can reduce soil erosion by a factor 2. 5 in the Sibalom river watershed.

Recommendations for LGUs and Agencies • Keep on measuring rain (NIA). Spread measurement locations over the whole watershed, also at locations higher in the mountains (>100 m, >500 m, >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