Stern, R., Soil and Irrigation Research Inst, P/Bag X79, Pretoria, 0001, South Africa
Laker, M.C., Dept. of Soil Sci. and Plant Nutrition, Univ. of Pretoria, Pretoria, 0002, South Africa
Van Der Merwe, A.J., Soil and Irrigation Research Inst, P/Bag X79, Pretoria, 0001, South Africa

Runoff plots (1.5 m2) were constructed at three sites where crusting, runoff and erosion are severe problems. The effect of surface application of phosphogypsum (PG), polyacrylamide (PAM) and mulch cover on runoff during natural rainstorms were studied. Runoff percentage from control (bare) plots in some rainstorms exceeded 90% of the rainfall. The annual runoff from control plots ranged between 33 and 73% of the annual rain. Mulch was highly beneficial in reducing runoff, indicating that seal formation restricted water penetration rather than hydraulic properties of the profile. PG reduced runoff to 0.15-0.82 of the untreated plots. The efficiency of PG in reducing runoff was inversely correlated with rainstorm intensity. The beneficial effect of PG in reducing runoff lasted throughout the rainy season. Its beneficial effect terminated during the consecutive season in the Irene site when the cumulative rain depth exceeded 700 mm. PAM treatment reduced annual runoff by two to three fold in comparison with the control treatment. The effect of PAM on diminishing runoff in the rainfall simulator studies was more pronounced than in the field conditions. © 1991, CSIRO. All rights reserved.