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Effect of Soil Slope and Rainfall Characteristics on Phosphorus in Runoff¹

ABSTRACT

Effects of soil slope length, degree of slope, soil cover, and storm size on soluble P released to runoff were investigated under controlled conditions, in 100- by 30- by 7.5-cm soil boxes, under a simulated rainfall. These effects were related to a simplified kinetic model, presented earlier, which was developed further to incorporate the above effects conceptually. An increase in slope length from 33 to 100 cm increased the average P released per unit area of soil. Most of this increase was explained by the effect of slope length on the water to soil ratio, which influences the kinetics of release. However, the effect of overland flow, which influences the mass of soil releasing P to runoff, also increased with the increase in slope length. An increase in percent soil slope also increased the P release, directly proportional to slope between 4 and 16%. This was explained, for a slope length of 100 cm, primarily by the effect of slope on raindrop impact. Soil covers, simulated by different-mesh screens, decreased the P release. Their effects were explained by the decreases in raindrop impact, which were determined by splash measurements. The effects of soil covers on P release increased with an increase in soil slope. The storm-average P concentration of runoff decreased logarithmically with an increase in storm size. These experimental results could be explained by terms in the conceptual model, which were based on an analogy with a physical soil erosion mechanism. Further work is needed, however, on longer slopes and under field conditions.

Key Words: rainfall-runoff-soil interaction • overlandflow-soil interaction • runoff mixing zone

¹ Contribution of the USDA-AR-SEA, Southern Plains Watershed and Water Quality Laboratory, Durant, OK 74701.

² Soil Scientists. The second author is a member of the Department of Agronomy, Oklahoma State University, Stillwater, but stationed at Durant under a cooperative agreement.

Received for publication December 24, 1980.