On Farm Soil Loss and Rainfall Runoff During Major Rainfall Events

One of the consequences of climate change is that rainfall events are often less frequent and more intense.  During the 1980s a series of studies relating to vetiver grass and soil and water conservation were carried out by Dr. G.M. Bahrad of Akola University in Maharashtra, India. Here is a summary of some of that work;

Drs. G.M. Bharad and B.C. Bathkal from PKV University in Akola, Maharashtra, India have provided the Network with another season’s data on the impacts of vetiver grass hedgerows on soil loss and surface runoff.

Figures 1 and 2 show the soil loss (total and maximum one day soil losses) and surface runoff from the plots over three years and Figure 3 compares these rates to the control plot (ACROSS) from the top 10% largest storms over the three years. The individual treatments are : ACROSS = across slope cultivation only; BUND = a graded (0.2%) earthen bund at a 1 meter vertical interval with contour cultivation; LEUCAENA = Leucaena leucocephala contour hedgerows at a 1 meter vertical interval with contour cultivation; and VETIVER = Chysopogon zizanioides contour hedgerows at a 1 meter vertical interval with contour cultivation. Plots are approximately 0.35 ha, slopes are less than 2%, soils are vertisols, climate is semiarid.

Referring to Figure 1, in all plots the majority of the soil loss occurred in year 1 (68% – 79% of the totals) with a substantial proportion of that loss coming from one storm event; year 1 was an unusually wet year. The numbers on top of the histograms are the total three year soil losses and the blue areas represent the soil lost in the one major storm event in year 1. As illustrated here, the adequacy of a soil conservation treatment must be judged not on the “average” event but on performance during exceptions to the average.

Figure 2 shows the total amount of surface runoff from all treatments. The numbers on the top of the histograms are surface runoff as a percent of the rainfall from storm events which caused surface runoff. The difference between vetiver and the across slope treatments represents almost 200 mm of rainfall. In year 2, a drought year, surface runoff from the vetiver plot was 55%, 35% and 41% less than from the across slope, leucaena, and bunded treatments, respectively.

Figure 3 is a followup on the comment made above regarding performance of conservation treatments during the extreme events. This figure represents the data from the top 10% largest storms over the three years. These storms also

represent the events which resulted in the greatest volumes of surface runoff. The figure shows soil losses and surface runoff from the leucaena, bunded and vetiver plots as a percentage of losses from the across slope “control” plot; the absolute amounts of soil loss and surface runoff for each treatment are given on top of the histograms. Across all three treatments, it was in this 10 percentile of storm events that one-half to two-thirds of the three year total soil loss occurred and about 50% of the total volume of surface runoff.

NOTE THAT THE HIGHEST LEVEL OF  RUNOFF IS FROM THE GRADED BUNDS.  CAN FARMERS AFFORD TO UNNECESSARILY LOSE THIS AMOUNT OF WATER?