SIMPLE MODEL TO ESTIMATE DAILY LATERAL DRAINAGE.

Lateral movement of soil solution could be a major cause for the spatial variability of crop yield within sloping landscapes. This study was an effort to develop a functional lateral downslope drainage model (LDDM) by expanding the Suleiman and Ritchie (2004) vertical drainage model. Within LDDM, the daily change of soil water content from a soil layer, due to lateral downslope drainage, was calculated by multiplying the drainable soil water [initial [theta] - drained upper limit [theta] ([theta]dul)] by hydraulic gradient and two coefficients. The two coefficients are C, which represents the soil hydraulic properties, and F, which accounts for the incoming soil water flow into a particular layer of the profile (Qi). To evaluate LDDM, soil water content profile and water table height were measured periodically at 15 locations along a transect along a sloping landscape during a lateral drainage cycle. The LDDM estimated lateral downslope soil water flow rate and the water table height reasonably well. The root mean square difference between estimated and measured daily lateral downslope soil water flow rate was 1.8 mm d^-1, and between estimated and measured cumulative weekly lateral downslope, soil water flow was 4.4 (first week), 16.3 (second week), and 8.7 (third week) mm.