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Most crop, hydrology, and water quality models require the simulation of evaporation from the soil surface. A model developed by J.T. Ritchie in 1972 provides useful algorithms for estimating soil evaporation, but it does not calculate the soil water redistribution resulting from evaporation. A physically-based model using diffusion theory, described previously by Suleiman and Ritchie in 2003, provides efficient algorithms for soil water redistribution and soil evaporation. However, the model is appropriate only for second stage drying when the soil in the entire profile being simulated is below the drained upper limit ([[theta].sub.DUL]) and no more drainage occurs due to gravity. This paper extends the Suleiman-Ritchie model for soil water contents higher than [[theta].sub.DUL] where soil evaporation rates are usually higher than second stage drying. New algorithms were developed for these wetter conditions that are functions of soil depth and the wetness of the near-surface soil. New model parameters were calibrated with data measured in laboratory soil column studies. The resulting model was integrated into DSSAT-CSM (Decision Support System for Agrotechnology Transfer Cropping Systems Model). Simulated soil evaporation rates and soil water contents obtained using the Suleiman-Ritchie model with the developed extensions and the previous DSSAT soil evaporation model were compared and evaluated with field measurements of soil water content during several drying cycles for parts of 3 yr in North Central Florida. Computed soil water contents from the model agreed well with the measured soil water contents near the surface, and provided more accurate estimations than the original DSSAT soil evaporation model, especially for the 5-cm surface layer. Abbreviations: [[theta].sub.DUL], Soil water content at drained upper limit ([cm.sup.3] [cm.sup.-3]); CSM, Cropping Systems Model; DSSAT, Decision Support System for Agrotechnology Transfer; ES, evaporation of water from soil surfaces (cm); ESR, Extended Suleiman and Ritchie soil evaporation modal; LAI, Leaf area index ([m.sup.2] [m.sup.-2]); MAE, metal average error; MicroWEX, Microwave, Water, and Energy Balance Experiment; ORD, Original Ritchie DSSAT soil evaporation model; SR, Suleiman and Ritchie method of soil evaporation using diffusion theory.

The climate in Georgia and other southeastern states of the United States is considered to be humid and the annual precipitation is usually greater than the annual potential evapotranspiration (ET). However, during several months of the year, supplemental irrigation is needed to prevent yield reducing water stress due to the temporal rainfall variability and sometimes due to long-term droughts. The Priestley-Taylor (PT) equation has been used operationally in Georgia to compute ET for irrigation scheduling because of its simplicity, its general acceptable performance in humid regions, and its limited input requirements. A recent study for a site in the humid southeastern United States found that PT overestimated ET and was less accurate than the FAO-56 Penman-Monteith (PM) among some of the approaches that were evaluated. The objective of this study was to assess the potential improvement that can be achieved by replacing PT with FAO-56 PM in Georgia and other southeastern states in a humid climate. More than 70 weather stations across Georgia are available as part of the Georgia Automated Environmental Monitoring Network. Nine representative sites, including Blairsville in a mountainous area and Savannah in a coastal area, were selected to assess the potential improvements that may be achieved by replacing PT with FAO-56 PM. Each site had at least 10 years of daily records that included minimum and maximum air temperature, solar radiation, wind speed, and vapor pressure deficit. PT underestimated the daily and monthly ET during the winter months in the central and southwestern areas and overestimated the daily and monthly ET during the summer months in the coastal and mountainous areas. For the warm season, i.e., April through September, PT slightly overestimated the cumulative ET in the central and southwestern areas, moderately for the mountainous area and severely for the coastal area. Based on these results, it is anticipated that the use of FAO-56 PM for estimating ET will standardize the ET calculations and improve irrigation efficiency in Georgia, especially for the mountainous and coastal areas. DOI: 10.1061/(ASCE)0733-9437(2007)133:2(175) CE Database subject headings: Georgia; Irrigation; Evapotranspiration.

This study compares saturated hydraulic conductivities (Ks) of three sandy soils such as coarse, medium and fine sand. The Ks was obtained using three different methods: empirical methods based on the grain size analysis, the relative effective porosity model (REPM), and breakthrough curve analyses based on tracer tests. Column drainage tests were performed to characterize the water retention properties of the samples, which are required in the REPM. Bench scale tracer tests with various conditions were conducted to obtain reasonable linear velocities of the samples using breakthrough curve analyses and then Ks was estimated using Darcy's law. For the REPM, the differences of Ks for the coarse and fine sand soils were less than one order of magnitude; however, the difference of Ks values between the empirical methods and the breakthrough curve analyses was larger than one order of magnitude. The comparison results suggest that the REPM can be a reliable method for estimating Ks for soil grains, and is cost effective due to its experimental simplicity. [ABSTRACT FROM AUTHOR]

The threat of land degradation by salinization in Jordan has been increasing over the last decades. Therefore, information about the response of local cultivars to salinity is needed to help farmers choose the most productive cultivars for areas with salt-affected soils. A recently released durum wheat cultivar Um Qais (Triticum turgidum subsp. durum) has shown to be productive under normal conditions but to date there are no known studies on its tolerance to salinity. Two experiments were conducted to investigate the response of Um Qais cultivar to salinity. A field experiment was carried out in the Jordan Valley, which is known for its hot, dry climate during the summer and low rainfall and moderate temperature during the winter. Three water salinity levels (S): S1 (2 dS m−1), S2 (4 dS m−1), and S3 (8 dS m−1) with three irrigation amounts (R) (control = 120% (R1), 100% (R2), and 70% (R3)) were used in the field. A greenhouse experiment was conducted using four levels of saline water (S): S1 (0.65 dS m−1), S2 (4 dS m−1), S3 (8 dS m−1), and S4 (10 dS m−1). In both experiments, the leaf area index (LAI) and canopy height were measured during three growth stages, tillering, flag leaf, and maturity. The number of grains, grain yield, and above-ground biomass were measured after harvesting while soil salinity and pH were measured every three weeks during the growing season. The results showed that the maximum reduction in yield was of the 28% in the field experiment when the average soil salinity was of 6.8 ± 1.1 (standard error) dS m−1 at the middle stages of the season. Significant changes were shown in the treatments of the field experiments for maximum LAI, number of grains, and aboveground biomass, but not for plant height. For the greenhouse treatments, about 60% of the maximum grain yield was obtained when the average soil salinity was 9.94 ± 1.89 dS m−1 at the middle stage. Grain yield was the most sensitive parameter to the increase in soil salinity during the season. According to the findings of both experiments, Um Qais can be cultivated in moderately saline soils. [ABSTRACT FROM AUTHOR]