Organized variability of surface runoff responses across neighboring hillslopes in Iowa

- Twelve neighboring agricultural experimental hillslopes were monitored in Iowa, USA.
- The hydrographs from these hillslopes have similar shapes but different magnitudes.
- This shows that small-scale surface runoff responses are linearly organized in space.

SummaryCharacterizing the spatial and temporal variability of small scale runoff responses is essential to distributed hydrologic modeling. To explore the variability of runoff responses, we analyzed surface runoff hydrographs from 12 neighboring hillslopes in central Iowa, USA that were observed for 72 runoff events over a four-year period. These agricultural experimental hillslopes receive various prairie filter strip treatments and drain areas ranging from 0.48 to 3.19 ha. The distances between them vary from tens of meters to about 3 km. We compared the hydrographs from the remaining 11 hillslopes to the hydrograph at the benchmark hillslope (i.e., hillslope B6 with no treatment). The results showed that: (1) for any individual event in which noticeable surface runoff occurred, the hydrographs from these hillslopes had similar shapes but different magnitudes; (2) for any paired hillslopes, the shape similarity persisted, but the scaling factor (the regression slope between two flow series) changed across events; and (3) for any runoff event, no simple relationship exists between the spatial variation of the scaling factor and the slope, slope length, area, and prairie strip width at the footslope of the hillslopes. Interestingly, we found that for 9 out of the 11 paired hillslopes, 40-70% of the temporal variation in the scaling factors can be explained by the antecedent wetness condition and the maximum hourly rain accumulation. These results suggest that the small-scale surface runoff responses are spatially variable but organized linearly, i.e., shape similarity (or linearity) in space is another feature of the small-scale runoff process. This phenomenon seems to result from the spatial vicinity and small-scale spatial variability of rainfall intensity and antecedent soil moisture.