Studies of crop response to water and salt stress vary either salinity with a high leaching fraction or irrigation in the absence of salinity to isolate and quantify the effects of the two types of stress. Under deficit irrigation with saline water, a water conserving practice, the crop experiences simultaneous matric and osmotic stress, and it is not known if experiments designed to isolate stress effects may be used to predict crop response to simultaneous stresses. Thus, a study was conducted wherein yields were determined under varying levels of salinity and irrigation. Corn (Zea mays L.) and melon (Cucumis melo L.) were grown at the Arava Research and Development Farm in Yotvata, Israel, and alfalfa (Medicago sativa L.) at the Utah Power & Light Research Farm in Huntington, UT. Corn and melon plots were drip irrigated at six ratios of potential evapotranspiration ranging from 0.2 to1.7 in combination with four salinity levels. Alfalfa was irrigated with water of 0.2 and 4.0 dS m−1 from a line‐source sprinkler. For all three crops, the salinity treatments consisted of a control treatment with a salinity level less than published salt‐tolerance thresholds. Interactive effects of salinity and water stress were not observed in these field experiments. At low irrigation levels (≈70% of potential evaporation), yields were unaffected by the salinity level. At the higher irrigation levels, the salinity level caused significant differences in yield. Yield data were fit to piecewise linear models that emphasized the limiting nature of the effects of salt and water stress.

[1] An accessible solution capable of reliably predicting plant-environmental interrelationships for variable species, climates, soils, and management options is a necessary tool for creating sustainable agriculture and environmental preservation. A mechanism-based analytical solution, the first of its kind that considers multiple environmental variables and their combined effects on plant response, was developed and tested. Water uptake by plants, water and salt leakage below the roots, and yield are calculated by solving for transpiration in a single mathematical expression according to limitations imposed by root zone salinity and water status. Input variables include the quantity and salinity of applied water, terms for plant sensitivity to salinity and to water stress, potential evapotranspiration, and soil hydraulic parameters. Where water was not limiting, regression of predicted versus measured data resulted in r 2 = 0.96 with slope of 0.937 and intercept of 0.033 (not different from 1 and 0 at 99% confidence), where irrigation varied and salinity was not limiting the r 2 = 0.94 with slope of 0.906 and intercept of 0.044 (not different from 1 and 0 at 99% confidence), where both salinity and water levels varied r 2 = 0.94 with slope of 0.966 and intercept of 0.033 (not different from 1 and 0 at 99% confidence). Application of the model for agricultural and environmental management and economic analysis is discussed. For example, a farmer in the Arava in Israel where irrigation water salinity is high (electrical conductivity of 3 dS m À1 ) cannot expect to reach greater than 70% of the potential yield for a pepper crop with any amount of irrigation. By choosing melon, the farmer can achieve 90% of potential yield with the same quality and quantity of water.Citation: Shani, U., A. Ben-Gal, E. Tripler, and L. M. Dudley (2007), Plant response to the soil environment: An analytical model integrating yield, water, soil type, and salinity, Water Resour. Res., 43, W08418,

At the field scale, because of the relatively large number of observation points required to estimate the spatial distributions of hydraulic properties, application of inversion procedures must be based on a characteristic of the flow that for a given point in the field can be measured with relative ease, and can be used as input for the inversion procedure. Suggesting the use of infiltration data for this purpose, and recognizing the fact that a basic requirement for the design of a transient flow experiment is that the resulting inverse problem be sufficiently well posed to allow solution, this paper addresses the question under which circumstances the resulting inverse problem is well posed, utilizing the concepts of identifiability, uniqueness, and stability. For this purpose, three different models of the soil hydraulic properties were analyzed. The main conclusion of this study is that when infiltration data measured at the Darcy scale are used as input for the inversion procedure, the inclusion of prior information on a single measurable parameter, the saturated conductivity Ks, in the estimation criterion will enhance the likelihood of uniqueness and stability of the inverse solution provided that the structure of the hydraulic model is sufficiently simple. However, since for a particular situation, it is impossible to determine a priori whether the resultant inverse problem is well posed or not, this must be carried out only a posterJori by solving the problem several times with different initial parameter estimates, accompanied by an analysis of the associated estimation errors. sure heads, etc.). Fundamental to the approach is that model parameters are determined in such a way that the ability of the flow model to reproduce the transient flow event is optimized.Most work on the development of inverse problem methodology in subsurface hydrology has been restricted to the case of saturated groundwater flow (e.g., see review by Yeh [!986]). Current applications of inverse problem methodology to unsaturated flow may be classified as "indirect" methods [see Neuman, 1973] because the problem is nonlinear in the parameters and must be solved iteratively by repeated simulations. "Indirect" methods may be distinguished from "direct" methods in which the inherent soil properties (e.g., transmissivities) are treated as unknowns and the response (e.g., heads) are treated as known parameters in the spirit of a Cauchy formulation. The applications to unsaturated flow have mostly involved one-dimensional analyses of inflow and outflow experiments on laboratory soil cores [Zachrnann et al., 1981[Zachrnann et al., , 1982Hornung, 1983;Kool et al., 1985;Parker et al., 1985]. While laboratory experiments are more accurate and generally more convenient than in situ field experiments, the utility of soil properties determined on typically small core samples for predicting in situ behavior is in many cases questionable. Dane and Hruska [!983] and Kool et al. [1987] determined soil hydraulic properties by inversion of in sit...

Plant roots consist of various functional zones; the root cap and beyond the lateral root formation region have very low water permeability due to immature water conduit and root suberization, respectively. The root hair zone, which is located between these regions, is the most permeable zone, where both radial and axial conductivities are high, and has been suggested to play a role in water uptake enhancement. The conventional understanding of root hair function is that root hairs increase root surface area, thereby enhancing water and nutrient uptake. Yet, modeling the soil water status between root hairs shows that the soil water potential there reaches a value close to that of the root in a very short time. The corresponding low water content values within the inter‐root‐hair domain indicates limited mass water flow and ion diffusion rate toward the root. Consequently, we conclude that when the plant transpires (daytime), root hairs do not increase water and nutrient uptake by increasing root surface area. Instead we used both magnetic resonance imaging technology, for measurements and analysis of spatial and dynamic changes in water content in the rhizosphere, and numerical modeling to show that: (i) root hairs function mostly by water uptake through the root hair tip plane; (ii) the growth of root hairs, perpendicular to the root surface, expands the apparent diameter of the cylinder that is characterized by the root water potential, thereby increasing the effective surface area of the root for water uptake; and (iii) the growth of needle‐shaped root hairs requires minimal investment in biomass with less mechanical resistance compared with alternative strategies that require larger root diameter or root length.

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