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Abstract:Pollutant delivery through artificial subsurface drainage networks to streams is an important transport mechanism, yet the impact of drainage tiles on groundwater hydrology at the watershed scale has not been well documented. In this study, we developed a two-dimensional, steady-state groundwater flow model for a representative Iowa agricultural watershed to simulate the impact of tile drainage density and incision depth on groundwater travel times and proportion of baseflow contributed by tile drains. Varying tile drainage density from 0 to 0.0038 m À1 , while maintaining a constant tile incision depth at 1.2 m, resulted in the mean groundwater travel time to decrease exponentially from 40 years to 19 years and increased the tile contribution to baseflow from 0% to an upper bound of 37%. In contrast, varying tile depths from 0.3 to 2.7 m, while maintaining a constant tile drainage density of 0.0038 m À1, caused mean travel times to decrease linearly from 22 to 18 years and increased the tile contribution to baseflow from 30% to 54% in a near-linear manner. The decrease in the mean travel time was attributed to decrease in the saturated thickness of the aquifer with increasing drainage density and incision depth. Study results indicate that tile drainage affects fundamental watershed characteristics and should be taken into consideration when evaluating water and nitrate export from agricultural regions.

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Construction of small water-harvesting structures (WHS) across seasonal streams, in the vicinity of cultivated areas, has proved to be a viable technique of water harvesting in the Shiwalik foothills of northern India. This paper reports on the performance and impact of one such structure in this area. Sufficient rainwater was available for harvesting because 86 ± 5.4% of monsoon rainstorms were runoff producing, resulting in collection of 29 ± 5% of the total monsoon rainfall. Out of this harvested water around 62% was available for irrigation at the start of the Rabi season (i.e., the dry season starting around 1 November). With time the water losses from the WHS declined from 66% in the first year of construction (1985) to 18% in 1996, because of sealing of soil pores from siltation of the reservoir bed. The life of the WHS was estimated to be 65 years at the observed average siltation rate of 47 Mg/ha/year. After construction of the WHS, the area under cultivation increased from 17 ha to 20 ha and the cropping intensity by 170 to 200%. The Rabi crop yields were approximately doubled. Assured availability of water and fodder increased milk production by 103%. Additional annual income of Rs. 7,015 per year was generated through fish culture. A rise in the groundwater level and higher growth of vegetation around the WHS were also observed. On the basis of increased crop production alone, the WHS proved to be an economically viable venture resulting in a benefit:cost ratio of greater than one. Socioeconomic considerations in the management of the WHS were also studied and showed that community involvement was essential to the success of this project.