Hortonian runoff was measured in the laboratory from uniform slopes of lengths of 1.5, 3.0, and 6.0 m for steady, high-intensity rainstorms with durations of 1·0 to 7.5 min. A clear reduction in runoff per unit slope length was found as slope lengths were increased. This effect becomes more pronounced with decreasing storm duration. The runoff data were used to validate a simple process-based model that combines the Philip-two-term infiltration equation with the kinematic wave overland flow principle. The predicted and experimental results agreed well. Laboratory findings were extrapolated with the aid of the model to slopes and rainfall durations similar to those found under West African conditions. The calculated reduction of runoff per unit length is similar to reported observations. Thus, this process-based model can largely explain the phenomenon of runoff reduction with increasing slope length.@en

Hortonian runoff was measured from plots with lengths of 1.25 and 12 m, and at watershed level for rainstorms during the 1996 rainy season in cental Cote d'Ivoire, Africa. A clear reduction in runoff coefficients was found with increasing slope lengths, giving order of magnitude differences between runoff measurements at point level (1 m2: 30-50% of total rain) and watershed level (130 ha: 4% of total rain). Runoff reduction from 1.25 and 12 m slopes was reproduced for each major runoff-producing rainstorm at two different sets of plots, but the reduction was erratic for rainfall events which produced little runoff. In addition, runoff reduction varied wildly from one rainstorm to the next. In the analysis, we show that the spatial variability of runoff parameters causes the erratic behaviour during rainstorms with little runoff. During the more important, larger runoff-producing events, which give 78% of total runoff, the temporal dynamics of the rainfall-runoff process determine the reduction of runoff coefficients from longer slopes. A simple infiltration/runoff model was used to simulate the field results, thereby confirming the importance of rainfall dynamics as an explanatory factor for measured reduction of runoff coefficients.@en

Discharge measurements over a range from 1 L/s to as low as 0.03 L/s require special instruments. A flowmeter was designed on the basis of theoretical analysis and two major design criteria: (1) the flowmeter should be accurate and flexible in monitoring the full range of indicated discharge rates, and (2) instantaneous measurements of hydrographs should be possible. Furthermore, the relationship between the observed water level in the flowmeter and the actual discharge was linearized to the maximum extent possible. This article describes the design and test results. The calibration data from four flowmeters show that the design fulfills the above criteria and is replicable.@en

Recent publications from field and simulation studies indicate that runoff per unit area decreases as the length of the slope being observed is increased. This scale effect has been observed and documented for slopes with a uniform infiltration capacity as well as for slopes along which infiltration capacity is variable. This paper presents the design and testing of a laboratory flume for the study of the processes that lead to this scale effect, particularly for the case of slopes covered with crops. The features of the flume include reliable experimental control of soil water content prior to rainfall, high intensity rainfall without erosion, uniform crop growth along the slope, and the option of varying the infiltration rate independently of rainfall and soil characteristics.@en