COMPARISON OF DRAINAGE UNDER STEADY RAINFALL VERSUS FALLING WATER-TABLE CONDITIONS

Numerical solutions of Richards' equation were used to compare the ste ady state rainfall ease to the falling water table ease of water flow to shallow drains in a uniform sand profile and in a layered silt loam soil. Equipotentials, streamlines, and velocity distributions were ca lculated. For saturated flow the velocity near the drain was 200 to 25 0 times the velocity near the midpoint between drains, and the drain o utflow rate was less than 10% of the saturated hydraulic conductivity of the soil profile. For a given drain flow rate the steady rainfall c ase always showed greater flow from areas further removed from the dra in than did the falling water table ease. Flow in the capillary fringe above the water table was generally more horizontal for the falling w ater table case than for the steady rainfall case. Flow in the zone ab ove the capillary fringe tended to be more vertical. With shallow drai ns, and especially in clay and silty clay soils, the capillary fringe may extend to near the soil surface so that most of the flow above the water table may have a significant horizontal component.