Rainfall Intensity Effects on Runoff and Sediment Losses From a Colorado Alfisol

Abstract

For the Front Range region of Colorado, quantifying rainfall partitioning under current and/or proposed farming practices and changing precipitation patterns is the first step to understanding how to efficiently conserve water and soil resources to meet crop water demands. We hypothesize that processes controlling runoff, sediment, and agrichemical losses would be better understood and predicted if variable rainfall intensity patterns derived from natural rainfall were used in rainfall simulation studies to evaluate runoff and sediment losses from conventionally tilled, intensively cropped Alfisols in the north-central region of Colorado. Our objective was to quantify the effects of constant (Ic) and more realistic, observed variable (Iv) rainfall intensity patterns (most frequent, Ivf; extreme, Ive) on runoff (R) and sediment (E) losses from a Fort Collins sandy clay loam (Aridic Haplustalf) cropped to conventional-till corn (Zea mays L.). Five treatments were evaluated: three Ic treatments (20, 47, and 65 mm h-1) and two Iv treatments representing the most common and most extreme events occurring in the region. Field plots, each 2 m wide by 3 m long, were established on each treatment. Each 6 m2 plot received simulated rainfall (Ic or Iv) for 60 min. For Ic events, increasing Ic 2.3-fold (20.3 vs. 47.3 mm h-1) and 1.4-fold (47.3 vs. 65.0 mm h-1) increased runoff 5.9- and 2.4-fold; sediment 4.9- and 4.1-fold; Rmax 5.8- and 1.9-fold; and increased Emax 3.1- and 3.9-fold, respectively. For Ivf vs. Ic (20 mm h-1) events, runoff, Rmax, and sediment losses were similar. However, Emax values for Ivf events were 33% less than those for Ic events. Also, tRmax and tEmax for Ivf events were 40-min (3-fold less) and 33-min (2.2-fold less) earlier than those for Ic (20 mm h-1) events. For Ive vs. Ic (65 mm h-1) events, runoff losses were similar. However, Ive events had 1.7-fold more sediment, 28% higher Rmax, and 2.9-fold higher Emax losses than those for Ic (65 mm h-1) events. Also, tRmax and tEmax for Ive events were 32-min (2.1-fold less) and 28-min (2.1-fold less) earlier than those for Ic (65 mm h-1) events. For Ivf vs. Ive events, runoff, Rmax, sediment, and Emax values for Ive events were 11.7-, 16.1-, 36- and 48-fold greater than corresponding values for Ivf events, respectively. Surface sealing, expressed as dINF (maximum infiltration-minimum infiltration), increased 19-fold as Ic increased 3.3-fold. The Iv patterns had at least 52% greater dINF values than corresponding values for Ic patterns; dINF values for Ive events were 3.1-fold greater than those for Ivf events. Our results show the pronounced effect of rainfall intensity patterns, particularly variable intensity patterns, on detachment and transport processes controlling runoff and sediment losses. A more accurate measure of rainfall partitioning, soil detachment, and sediment transport was obtained when Iv patterns derived from natural rainfall were utilized and evaluated.