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Y/T ϭ m/T max [2] Water use efficiency (WUE) represents a given level of biomasswhere Y is the total dry matter production, T is the or grain yield per unit of water used by the crop. With increasing transpiration, m is a coefficient, and T max is the daily concern about the availability of water resources in both irrigated free water evaporation. Water use efficiency is estiand rainfed agriculture, there is renewed interest in trying to develop mated using the total water use (ET) from a crop suran understanding of how WUE can be improved and how farming face, which includes evaporation from soil and plant systems can be modified to be more efficient in water use. This review components because of the difficulty in separating evapand synthesis of the literature is directed toward understanding the oration from transpiration. role of soil management practices for WUE. Soil management prac-Changes in WUE can be manifested through soil mantices affect the processes of evapotranspiration by modifying the available energy, the available water in the soil profile, or the exchange agement practices via the components of the surface rate between the soil and the atmosphere. Plant management pracenergy balance: tices, e.g., the addition of N and P, have an indirect effect on water ET ϭ R n Ϫ G Ϫ H Ϫ P [3] use through the physiological efficiency of the plant. A survey of the literature reveals a large variation in measured WUE across a range where ET is evapotranspiration, R n is net radiation, G of climates, crops, and soil management practices. It is possible to is soil heat flux, H is sensible heat flux, and P is photoincrease WUE by 25 to 40% through soil management practices that synthetic flux. These terms can be expressed in a variety involve tillage. Overall, precipitation use efficiency can be enhanced of units (e.g., W m Ϫ2 and KJ m Ϫ2 s Ϫ1 ). Soil management through adoption of more intensive cropping systems in semiarid practices impact WUE through changes in the energy environments and increased plant populations in more temperate and exchanges (R n , G, and H) and through the plant photohumid environments. Modifying nutrient management practices can synthetic (P) efficiency. These terms will affect the waincrease WUE by 15 to 25%. Water use efficiency can be increased through proper management, and field-scale experiences show that ter balance in the soil within a growing season and across these changes positively affect crop yield.growing seasons. Throughout this review, we will show where soil management practices modify the energy balance components.

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[1] Most measurement approaches for determining evaporation assume that the latent heat flux originates from the soil surface. Here, a new method is described for determining in situ soil water evaporation dynamics from fine-scale measurements of soil temperature and thermal properties with heat pulse sensors. A sensible heat balance is computed using soil heat flux density at two depths and change in sensible heat storage in between; the sensible heat balance residual is attributed to latent heat from evaporation of soil water. Comparisons between near-surface soil heat flux density and Bowen ratio energy balance measurements suggest that evaporation originates below the soil surface several days after rainfall. The sensible heat balance accounts for this evaporation dynamic in millimeter-scale depth increments within the soil. Comparisons of sensible heat balance daily evaporation estimates to Bowen ratio and mass balance estimates indicate strong agreement (r 2 = 0.96, root-mean-square error = 0.20 mm). Potential applications of this technique include location of the depth and magnitude of subsurface evaporation fluxes and estimation of stage 2-3 daily evaporation without requirements for large fetch. These applications represent new contributions to vadose zone hydrology.