Evapotranspiration (ET) can cause diel fluctuations in the elevation of the water table and the stage in adjacent streams. The diel fluctuations of water levels change head gradients throughout the day, causing specific discharge through near‐stream sediment to fluctuate at the same time scale. In a previous study, we showed that specific discharge controls the residence time of groundwater in streambed sediment that, in turn, exerted the primary control on NO3− removal from groundwater passing through the streambed. In this study, we examine the magnitude of diel specific discharge patterns through the streambed driven by ET in the riparian zone with a transient numerical saturated–unsaturated groundwater flow model. On the basis of a first‐order kinetic model for NO3− removal, we predicted diel fluctuations in stream NO3− concentrations. Model results indicated that ET drove a diel pattern in specific discharge through the streambed and riparian zone (the NO3− removal zones). Because specific discharge is inversely proportional to groundwater travel time through the NO3− removal zones and travel time determines the extent of NO3− removal, diel changes in ET can result in a diel pattern in NO3− concentration in the stream. The model predictions generally matched observations made during summertime base‐flow conditions in a small coastal plain stream in Virginia. A more complicated pattern was observed following a seasonal drawdown period, where source components to the stream changed during the receding limb of the hydrograph and resulted in diel fluctuations being superimposed over a multi‐day trend in NO3− concentrations. Copyright © 2013 John Wiley & Sons, Ltd.

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Quantifying the effects of anthropogenic processes on groundwater in arid regions can be complicated by thick unsaturated zones with long transit times. Human activities can alter water and nutrient fluxes, but their impact on groundwater is not always clear. This study of basins in the Trans-Pecos region of Texas links anthropogenic land use and vegetation change with alterations to unsaturated zone fluxes and regional increases in basin groundwater NO 3 − concentrations.Median increases in groundwater NO 3 − (by 0.7-0.9 mg-N/l over periods ranging from 10 to 50+ years) occurred despite low precipitation (220-360 mm/year), high potential evapotranspiration (~1570 mm/year), and thick unsaturated zones (10-150+ m). Recent model simulations indicate net infiltration and groundwater recharge can occur beneath Trans-Pecos basin floors, and may have increased due to irrigation and vegetation change. These processes were investigated further with chemical and isotopic data from groundwater and unsaturated zone cores. Some unsaturated zone solute profiles indicate flushing of natural salt accumulations has occurred. Results are consistent with human-influenced flushing of naturally accumulated unsaturated zone nitrogen as an important source of NO 3 − to the groundwater. Regional mass balance calculations indicate the mass of natural unsaturated zone NO 3 − (122-910 kg-N/ha) was sufficient to cause the observed groundwater NO 3 − increases, especially if augmented locally with the addition of fertilizer N. Groundwater NO 3 − trends can be explained by small volumes of high NO 3 − modern recharge mixed with larger volumes of older groundwater in wells. This study illustrates the importance of combining long-term monitoring and targeted process studies to improve understanding of human impacts on recharge and nutrient cycling in arid regions, which are vulnerable to the effects of climate change and increasing human reliance on dryland ecosystems.