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Agricultural Practices Influence Flow Regimes of Headwater Streams in Western Iowa

National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011

View larger version (44K): [in a new window]   Fig. 1. Maps of the four watersheds comprising the Deep Loess Research Station. The weather station in the southwest part of W3 included the evaporation pan, one of the rain gauges, and provided the temperature data used to calculate monthly PET.

View larger version (32K): [in a new window]   Fig. 2. (A) Average cumulative precipitation for four watersheds from water years 1971 through 1995 (left axis). Cumulative deviations from the average are given on the right axis. Watershed 1 and W2 received more cumulative precipitation. However, (B) daily rainfall frequency curves show W3 and W4 received more precipitation from the heaviest storms.

View larger version (42K): [in a new window]   Fig. 3. Monthly observations of (A) precipitation, (B) runoff, (C) baseflow, and (D) total discharge from W1 between 1971 and 1995 (dotted lines). The annual and multi-year cyclic variations determined through autoregressive modeling (given in tables) are also plotted (heavy lines). For baseflow and total discharge, the full model with an autoregressive term is also plotted (thin line).

View larger version (23K): [in a new window]   Fig. 4. (A) Cumulative surface runoff and (B) runoff-frequency curves for four experimental watersheds during water years 1971 through 1995.

View larger version (24K): [in a new window]   Fig. 5. (A) Cumulative baseflow and (B) baseflow-frequency curves for four experimental watersheds during water years 1971 through 1995.

View larger version (31K): [in a new window]   Fig. 6. Baseflow from the four watersheds, expressed as a 365-d moving average. Each date is plotted as the average baseflow during the prior 365 d, removing seasonal variation from the record. Four drought periods, indicated by upward-pointing arrows, each showed a slower recovery of baseflow from the conventionally tilled watersheds (W1, W2) than the conservation watersheds (W3, W4).

View larger version (34K): [in a new window]   Fig. 7. Daily baseflow from the four watersheds during the 1971–1995 experimental period (thin dotted lines). Annual and long-term cycles identified by autoregressive models (solid lines) show greater intra-annual variation under conservation treatments (W3, W4). With autocorrelation terms, the models account for 99% of the variation in baseflow. The full models are not plotted because they are not distinguishable from the data at this scale.

View larger version (23K): [in a new window]   Fig. 8. (A) Cumulative total discharge and (B) discharge-frequency curves for four experimental watersheds during the 1971–1995 experimental period.