Length and Slope Effects on Runoff from Sodium Dispersed, Compacted Earth Microcatchments

The effects of microcatchment slope and length on runoff and erosion rates were studied under 18 rain storms to get information for design of water harvesting systems. Two replicates of a two‐factor experiment, including 1, 5, 10, and 15% slopes and 3‐and 6‐m lengths, were built on a gravelly sandy clay loam (mixed mineralogy) using 11.2 t ha −1 of NaCl mixed into the surface 2 to 5 cm of soil followed by compaction with a 6‐t roller after a heavy rain. Both slope and length had effects on runoff significant at the 0.1% probability level but the effect of rainfall intensity was not significant. The best fit linear model of runoff, Q , (cm) vs. rainfall, P , (cm) was Q = 0.858( P − 0.34) which explained about 98% of the variability in runoff on individual plots for individual storms but which explained no differences in runoff among treatments. A nonlinear model incorporating slope, s , (m/m) and length, x , (m) accounted for about 96% of the variability in average total runoff among treatments. This model, Q = ( s 0.0453 x −0.183 + 0.183)( P − 0.339), shows that the important differences in runoff among treatments can be explained by slope and length alone for our conditions. Water quality analyses indicated that only a minor salinity problem but slight to moderate permeability problems might be expected. However, no apparent problems have arisen in 12 yr of grape ( Vitis vinifera ) and fruit tree ( Prunis sp.) cultivation in a water harvesting system using our catchment on the same soil.