Characteristics and modeling of runoff hydrographs for different tillage treatments

Surface runoff rate is a critical variable in determining the rate of soil erosion and sediment transport. Rainfall and runoff data at I-min intervals from an experiment site at Khon Kaen, Thailand, were used to test a three- parameter runoff model originally developed for bare plots in relation to s oil erosion studies. The site has a sandy soil with a slope of 3.6%. Plot l ength and width were 30 and 5 m, respectively. Four tillage treatments with three replicates each were considered: up- and down-slope cultivation, two contour cultivation treatments with tillage depth of 25 and 50 cm, respect ively, and no tillage. Runoff data for 200 individual runoff hydrographs sh owed that runoff amount and peak runoff rate for the no tillage treatment w ere significantly less than those for other treatments at the site. On aver age, runoff amount and peak runoff rate for the no tillage treatment were 3 7 and 44%, respectively, of those for the up- and down-slope cultivation. R esults for contour cultivation practices are between the two extremes, alth ough the water retention was not greater with greater tillage depth as we o riginally thought would be the case at the site. For these 200 runoff event s for the four treatments, the model for runoff hydrographs worked well, wi th an average coefficient of efficiency of 0.90 and an average standard err or of 0.88 mm h(-1). The model performance is particularly good for large s torm events dth high volumetric runoff coefficient. The three model paramet ers vary considerably from event to event and from treatment to treatment. The initial infiltration amount was found to be inversely related to prior 10-d rainfall at the site; the spatially averaged maximum rate of infiltrat ion ran be related to the maximum retention or the Soil Conservation Servic e (SCS) Curve Number, and the hydrologic lag time is least variable among d ifferent storm events and tillage treatments, but tends to decrease,vith pe ak runoff rate.