[1] Public subsidies for promoting the adoption of water-conserving on-farm irrigation technologies are frequently cited as means for making additional water available for higher-valued uses in the water-scarce western United States. On the basis of an agroeconomic model reflecting conditions in northeastern Colorado, hypothetical conservation subsidy policies are analyzed with regard to their effects on hydrologic and agronomic factors such as irrigation water delivery, consumptive use, and return flows, as well as on economic factors including crop mix, input use, irrigation technology choice, and agricultural net returns to water. The cost-effectiveness of different subsidy arrangements for generating delivery reductions is also assessed, and implications for their implementation are derived. In contrast to assumptions underlying federal policies, the results confirm and extend earlier academic findings that subsidy policies are unlikely to provide real water conservation under many frequently encountered river basin conditions. Citation: Scheierling, S. M., R. A. Young, and G. E. Cardon (2006), Public subsidies for water-conserving irrigation investments: Hydrologic, agronomic, and economic assessment, Water Resour. Res., 42, W03428,

Two root water uptake terms, representative of the two major types commonly employed in soil‐based models of water and solute movement, were evaluated. Simulations were run to test the sensitivity of the two terms to salinity and water content and to investigate the details of their respective form and function. The two root water uptake terms tested were: (i) a mechanistic equation based on Darcy's law (Type I), and (ii) an empirical equation relating soil water potential to relative water uptake (Type II). The Type I term was insensitive to salinity where no reduction in transpiration was shown for increasing irrigation water salinity from 0.0 to 6.0 dS/m and applying water equal to potential transpiration. The Type II term was sensitive to salinity and showed a 35% reduction in water uptake by increasing water salinity from 0.0 to 6.0 dS m−1 and applying water equal to potential transpiration. Predicted reduction in water uptake due to matric potential was of the same magnitude as that due to salinity. The Type I term resulted in abrupt shifts in water uptake between full and zero transpiration, occasionally resulting in long periods of computed zero transpiration, uncharacteristic of conditions in the field. It was concluded that the Type I water uptake term may not be appropriate for models incorporating root water uptake, particularly under saline conditions.

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High concentrations of B and Se found in some arid environments are detrimental to sustainable agriculture. Vegetation management may be a remediation strategy designed to reduce soil B and Se concentrations to nontoxic levels. Two separate field experiments were conducted to study B and Se uptake in four different plant species grown in soil containing high concentrations of B (water‐extractable B ranging from 1–10 mg kg−1 soil) and Se (total soil Se ranging from 0.1–1.2 mg kg−1 soil). The four species were Brassica juncea L. Czern and Coss (Indian mustard), Festuca arundinacea Schreb cv. Fawn (tall fescue), Lotus corniculatus L. (birdsfoot trefoil), and Hibiscus cannibinus L. (kenaf). In the 1990 experiment, there were no differences in either tissue B or Se concentrations among the species. The mean tissue concentration was 105 mg B kg−1 dry matter (DM) and 0.75 mg Se kg−1 DM, respectively. In the 1991 experiment, mean shoot tissue concentrations of B ranged from a low of 96 mg kg−1 DM in tall fescue to a high of 684 mg B kg−1 DM in leaves from kenaf. Indian mustard accumulated the greatest amount of Se (>1 mg Se kg−1 DM), while the mean tissue concentration among the other three species was 0.36 mg Se kg−1 DM. For both experiments, soil samples were taken prior to planting and after harvest for each species to a depth of 0 to 30 and 30 to 60 cm, and analyzed for water‐extractable B and total Se. Summary data from all species indicated that extractable soil B and total Se concentrations were reduced between 0‐ to 60‐cm soil depth by 52 and 48% in 1990, and by 24 and 13% in 1991, respectively. Planting any of the four species tested in B‐laden soils may lead to a reduction in both B and Se concentrations in the soil.

Manganese solubility has become a primary concern in the soils and water supplies in the Alamosa River basin, Colorado due to both crop toxicity problems and concentrations that exceed water quality standards. Some of the land in this region has received inputs of acid and trace metals as a result of irrigation with water affected by acid mine drainage and naturally occurring acid mineral seeps. The release of Mn, Zn, Ni, and Cu following saturation with water was studied in four soils from the Alamosa River basin. Redox potentials decreased to values adequate for dissolution of Mn oxides within 24 h following saturation. Soluble Mn concentrations were increased to levels exceeding water quality standards within 84 h. Soluble concentrations of Zn and Ni correlated positively with Mn following reduction for all four soils studied. The correlation between Cu and Mn was significant for only one of the soils studied. The soluble concentrations of Zn and Ni were greater than predicted based on the content of each of these metals in the Mn oxide fraction only. Increases in total electrolyte concentration during reduction indicate that this may be the result of displacement of exchangeable metals by Mn following reductive dissolution of Mn oxides.