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145The United Nations of Food and Agricultural Organization (FAO) proposed a methodology for computing crop evapotranspiration (ET o ) and crop coefficient (K c ) (Doorenbos and Pruitt 1977). These coefficients depend on several factors including crop type, stage of crop growth, canopy height and density (Allen et al. 1998). To schedule irrigation properly, an accurate and standard method to estimate ET o to predict crop water requirements, was stated by several authors (Chiew et al. 1995;Allen 1996). A great number of models was developed to estimate ET o for use in environments that lack direct ET o measurements Pruitt 2004, Gavilán et al. 2006). A major complication in ET o estimation using these models is the requirement for meteorological data that may not be easily available. This restriction at times prohibits use of more accurate models, and necessitates the use of models that have less demanding data requirements.An international scientific community has accepted the FAO56 Penman-Monteith (FAO56PM) model as the most precise one for its good results when compared with other models in various regions of the entire world (Chiew et al. 1995, Garcia et al. 2004, Gavilán et al. 2006. Estimation of reference ET o by globally accepted FAO56PM (Allen et al. 1998) requires the weather parameters like maximum and minimum temperature, solar radiation, sunshine hours, wind speed, relative humidity. However, for many locations, as is the case for Jordan, such meteorological variables are often incomplete and/ or not available. Furthermore, no published studies have examined the applicability of ET o models across Jordan, a country with a great gradient in temperature and precipitation. Moreover, the local calibration and validation of other models is more important in semiarid and arid regions than the temperate climate because most of these models were calibrated and validated in temperate environment (Dehghani Sanji et al. 2003). The objectives of this study were to (1) assess the performance of simpler models that require less readily available data against FAO56PM, and (2) to determine models performance across spaces in Jordan, focusing on arid versus semiarid environment. Evaluation of evapotranspiration models for estimating daily reference evapotranspiration in arid and semiarid environments O.E. MohaweshDepartment of Plant Production, Faculty of Agriculture, Mu'tah University, Al-Karak, Jordan ABSTRACT Daily outputs from eight evapotranspiration models were tested against reference evapotranspiration (ET o ) data computed by FAO56PM to assess the accuracy of each model in estimating ET o . Models were compared at eight stations across Jordan. Results show that Hargreaves modified models were the best in light of mean biased error (MBE), root mean square error (RMSE) and mean absolute error (MAE). The MBE, RMSE, and MAE values ranged from -1.47 to 0.81, 3.87 to 1.14 and 0.87 to 3.15 mm/day for HarM1, and from -1.45 to 0.89, 1.08 to 3.91, and 0.85 to 3.16 mm/day for HarM2, respectively, which would make it th...

The objectives of this study are to compare reference evapotranspiration (ET 0 ) computed using different forms of the Hargreaves (HG) equation with the standard FAO56PM; and to compare ET 0 computed using the FAO56PM with limited data with the standard FAO56PM complete data set method. For this purpose, climatic data from 12 weather stations covering an area of *90 km 2 of predominantly arid and semi-arid conditions are used. Ratios of ET 0 estimated using the updated HG equation were closer to 1 when compared with the HG equations and ET 0 -est. Also, RMSE, ME and R 2 ranged from 0.235 to 0.526 mm day 71 , 70.526 to 0.383 mm day 71 , and 0.945 to 0.989, respectively. Therefore, the updated HG equation can be used for ET 0 estimates instead of using the FAO56PM method with acceptable accuracy. In addition, it was found that the standard FAO56PM can be used with estimated solar radiation and humidity from temperature data and estimates of mean wind speed. In conclusion, this study produced two accurate methods that can be used to calculate monthly ET 0 in conditions characterized by limited data, arid and semi-arid environments.

2011) Effect of irrigation regimes on water use efficiency and tomato yield (Lycopersicon esculentum Mill.) grown in an arid environment, Archives of Agronomy and Soil Science, 57:1, 105-114, This study was conducted to determine the influence of irrigation regimes on yield of two tomato cultivars -TY-DANA and GS12. Experimental treatments were three irrigation intervals (2, 4, and 6 days), and three irrigation quantities (50%, 75%, and 100% of pan evaporation [Ep]). The total yield increased with increasing amount of irrigation water for both cultivars. The total yield increased about 10% and 17% for 100% of Ep treatment compared with 50% Ep treatment for TY-DANA and GS12 cultivars, respectively. The 100% Ep treatment showed the lowest irrigation water use efficiency (IWUE) (16.7 and 10.4 kg m 73 ), while 50% Ep showed the maximum IWUE (23.9 and 17 kg m 73 ) for TY-DANA and GS12 cultivars, respectively. The average fruit weight for 100% Ep treatment was usually higher than the other treatments. Generally, yield decreased using 50% Ep, however, the benefits of increasing IWUE and saving around 50% of irrigation water will help to grow crops in more areas and in managing the limited water resources in arid environments.

Deficit irrigation has been widely investigated as a valuable strategy for dry regions where water is the limiting factor in crop cultivation. Soil moisture can be one of the important factors that influence root-knot nematode (RKN) disease development. To determine how different levels of irrigation can affect disease development, irrigation regimes ranging from 20% to full field capacity (FC) were applied to tomato and eggplant plants inoculated with Meloidogyne javanica (M. javanica) under controlled conditions. In addition, in vitro bioassays were done to evaluate the effect of water potential and soil moisture content on M. javanica viability in the soil and reproduction on plant hosts. The relative egg hatching percentage decreased significantly with decreasing water potential from -0.1 to -1 MPa. The use of 80% irrigation level caused minor reductions in growth but significantly reduced nematode infection load. Nematode infection was reduced even further at lower levels of irrigation, however this also led to marked reductions in fresh and dry weights of the tomato and eggplants. Therefore, deficit irrigation could be used at a rate of 80% or 60% of FC to increase water use efficiency and reduce the level of RKN (M. javanica) infection without greatly reducing the growth performance of tomato and eggplant crops.