Geographic information systems-assisted soil water dynamics model for predicting spatial and temporal soil moisture variations on a catchment scale

A GIS [Geographic Information Systems]-assisted soil water dynamics model that predicts spatial and temporal soil moisture on a catchment scale was developed. The model was parameterized and validated using biophysical attributes of Bug-ong micro-catchment in Rizal, Claveria, Misamis Oriental, Philippines. The catchment had two general cropping systems, namely: alley cropping and forest. Alleys were planted with corn while hedgerows were banana. Some banana hedgerows were interspersed with Acacia mangium Willd. and Eucalytus deglupta Blume. Acacia mangium Willd and Gmelina arborea Roxb. of ages 10-12 years were planted in the forest. The gravimetric moisture contents were determined from February to March 2006 in 4 soil layers (0-10 cm, 10-20 cm, 20-40 cm, and 40-60 cm) at locations that represented different cropping systems. Rainfall time series was measured using tipping-bucket rain gauge with date logger. Daily Potential evapotranspiration was derived from Class A evaporation pan readings. Saturated infiltration rate was obtained using double-ring method. Other soil physical properties were measured using standard laboratory procedures. Vegetation were delineated using Global Positioning System. The model utilized Darcy's Law, water mass conservation equation, and PC Raster modeling Language. Primary data inputs to run the model were saturated infiltration rate, soil map, digital elevation map, vegetative cover, rainfall and evapotranspiration. Outputs of the model are maps that show soil water variations in the catchment calculated every 15 minutes. The model predicted soil moisture variations with a correlation coefficient between 0.85 to 0.95. Soil moisture variations were mainly influenced by cropping systems and rainfall. Soil moisture content at the upper soil layer were higher in the forest soil than in the alley cropping system but the opposite was true at deeper layers due to more roots at the upper layer of the alley-cropping system and increasingly more roots in the deeper layers of the forests. Soil water was within the available water range except during high rainfall rates when soil was almost saturated. The model can predict temporal soil moisture variation on a catchment scale and it can be used to identify areas within the catchment where soil water limits crop growth.