An evaluation of two hourly reference evapotranspiration equations for semiarid conditions

The methodology proposed by the Food and Agriculture Organization (FAO) (Doorenbos, J., Pruitt, W.O., 1977. Crop water requirements. FAO irrigation and drainage. Paper No. 24. FAO, Rome) and updated by Allen et al. (Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration. Guidelines for computing crop water requirements. FAO irrigation and drainage. Paper No. 56. FAO, Rome) for calculating crop water requirements is the most extended and accepted method worldwide. This method requires the prior calculation of reference evapotranspiration (ET_o). This study evaluates the FAO-56 and American Society of Civil Engineers (ASCE) Penman–Monteith (PM) equations for estimation of hourly ET_o under the semiarid conditions of the province of Albacete (Spain). The FAO-56 and ASCE equations (hourly time step) were compared against measured lysimeter ET_o values at Albacete for 13 days during the period of April–October 2002 and 16 days during April–October 2003. The average of estimated FAO-56 Penman–Monteith ET_o values was equal to the average of measured values. However, the average of estimated ASCE Penman–Monteith values was 4% higher than the average of measured lysimeter ET_o values. This method overestimated measured lysimeter ET_o values by 0.45 mm h⁻¹.Simple linear regression and error analysis statistics suggest that agreement between both estimation methods and the lysimeter was quite good for the province of Albacete.In this paper, the FAO-56 Penman–Monteith equation for calculating hourly ET_o values was more accurate than the ASCE Penman–Monteith method under semiarid weather conditions in Albacete.     

Lysimetric determination of muskmelon crop coefficients cultivated under plastic mulches

A trial was carried out at the lysimeter station in southern Italy on muskmelon crop cultivated with and without plastic mulch during spring–summer in 2001 and 2003. The objective of the experiment was to verify the reliability of the crop evapotranspiration (ET_c) estimate by means of the most recent update of the FAO Irrigation and Drainage Paper 56, in comparison with ET_c directly measured by two mechanical weighing lysimeters.Crop coefficients (K_c) were determined during different development stages based on lysimetric measures of ET_c and of the reference evapotranspiration (ET₀) estimated through the Penman Monteith and the Hargreaves methods. On melon crop cultivated without plastic mulch, corrected crop coefficients (K_c) following the last FAO Irrigation and Drainage Paper 56 procedures were well correlated with those measured from lysimeter and were as reliable as the ET_c estimate. In contrast, values of K_c proposed by FAO Irrigation and Drainage Paper 56 for crops grown with plastic mulch were meaningfully lowers than those measured from lysimeter, loading to an underestimation of water consumption. On muskmelon, cultivated with and without plastic mulch, it is necessary to adapt development phase duration, suggested by the FAO Irrigation and Drainage Paper 56, to the real phenology of the employed cultivar. An adaptation of the phenology to the real duration of the single phases is essential to avoid error in the estimate of ET_c.     

A wind-based qualitative calibration of the Hargreaves ET0 estimation equation in semiarid regions

This study evaluates the Hargreaves (HARG) equation for estimation of monthly ET₀ under the semiarid conditions of the middle Ebro River Valley (NE Spain). First, the Hargreaves equation was compared against measured lysimeter ET₀ values at Zaragoza for the period May 1997–October 2000. The average of estimated values was only 5.6% above the average of measured values. Later, the Hargreaves equation was compared against the FAO Penman–Monteith equation for monthly ET₀ estimation at nine locations. These locations can be grouped as non-windy (Alcañiz, Daroca and Tamarite) and windy (Almudévar, Ejea, Gallocanta, Monflorite, Sariñena and Zaragoza). Simple linear regression and error analysis statistics suggest that agreement between the two estimation methods was quite good for the windy locations. Average errors ranged between 2 and 5% for Almudévar, Ejea, Sariñena and Zaragoza, and between 7 and 10% for Gallocanta and Monflorite where some underestimation was observed. However, the agreement between the Hargreaves and FAO Penman–Monteith equations was lower for the non-windy locations. In this case, the Hargreaves equation overestimated ET₀ and average errors varied between 14 and 20%. According to these results, it is proposed that, under the semiarid conditions of this study, no local calibration would be required for windy locations (those where monthly average windspeeds above 2.0 m s⁻¹ are frequent), while a value of 0.0020 instead of the original 0.0023 should be used in the Hargreaves equation for non-windy locations. Further research should be undertaken to evaluate whether these results can be extended to other semiarid regions of the world.     

Water requirements of maize in the middle Heihe River basin, China

As part of an intercomparison study on crop evapotranspiration (ET_c), six methods for estimating ET_c have been applied to maize field in the middle Heihe River basin, China. The ET_c was estimated by the soil water balance and Bowen ratio-energy balance methods while the Priestley-Taylor, Penman, Penman-Monteith and Hargreaves methods were used for estimating the reference evapotranspiration (ET₀). The results showed that the trend of ET_c was very similar, while the differences were significant among the different methods. The variations of ET_c were closely related to the LAI as well as to the meteorological features. The ET_c for the Bowen ratio-energy balance, Penman, Penman-Monteith, soil water balance, Priestley-Taylor and Hargreaves methods totaled 777.75, 693.13, 618.34, 615.67, 560.31 and 552.07 mm, respectively, with the daily mean values for 5.26, 4.68, 4.18, 4.16, 3.79 and 3.73 mm day⁻¹. The Penman-Monteith method provided fairly good estimation of ET_o as compared with the Priestley-Taylor, Penman, Hargreaves methods. By contrast with the Penman-Monteith method, the Bowen ratio-energy balance and Penman methods were 25.8% and 12.0% higher, while the Priestley-Taylor and Hargreaves methods were 9.4% and 10.7% lower, respectively. Therefore, the Hargreaves and Priestley-Taylor methods were the alternative ET_c methods in arid regions of Northwest China.     

Developing crop coefficients for field-grown tomato (Lycopersicon esculentum Mill.) under drip irrigation with black plastic mulch

A 2 years field study was conducted to develop crop coefficients for field-grown tomato (Lycopersicon esculentum Mill.), a major irrigated crop in the Jordan Valley, under drip irrigation system with black plastic mulch. The area of the study field was 1.5 ha surrounded by many similar tomato fields. Actual crop evapotranspiration (ET_C) was measured using eddy covariance technique which distinguishes this study from other previous studies conducted in the Jordan Valley that relied on the old indirect approach for ET_C estimation based on the soil water balance.Grass reference evapotranspiration (ET_O) was determined by using the FAO Penman–Monteith method utilizing the agrometeorological parameters measured at the study site. The crop coefficient (K_C) was determined as the ratio of ET_C to ET_O. The tomato crop coefficients were determined following the FAO crop coefficient model. The average crop coefficient during the midseason growth stage (K_C mid) was 0.82 which is far below the adjusted FAO crop coefficient of 1.19 by about 31%. Also, the late season crop coefficient (K_C end) was much lower than the adjusted FAO crop coefficient of 0.76 by about 40%. Moreover, the weighted average crop coefficient over the entire growing season (K_C GS) was 0.69, which is about 36% lower than the FAO corresponding value. In fact, the low K_C values obtained reflect the effect of practicing both localized drip irrigation and plastic mulch covering. This study showed that there is a big difference between the reported FAO crop coefficients and the one measured in the filed using a precise approach. These exact updated values of crop coefficients will enhance future estimation of crop water requirements and hence irrigation management of tomato crop which is the major irrigated crop in the Jordan Valley.     

Radiation calibration of FAO56 Penman-Monteith model to estimate reference crop evapotranspiration in China

The standardized FAO56 Penman-Monteith model, which has been the most reasonable method in both humid and arid climatic conditions, provides reference crop evapotranspiration (ET_o) estimates for planning and efficient use of agricultural water resources. Net radiation is an important and site-specific component to determine ET_o. The empirical radiation estimation in FAO56 Penman-Monteith model was calibrated by observed solar radiation of 81 meteorological stations over China during 1971-2000, and measurements of net longwave radiation in the Tibetan Plateau. Results showed that Ångström formula based on simple annual linear regression coefficients of 0.20 and 0.79 yielded the least error for the preserved 30 validation stations, and are thus recommended for estimating solar radiation in China. The optimal calibration of net longwave radiation was based on Penman estimation combined with the minimum and maximum temperatures. The calibrated net radiation served as the basis to estimate ET_o accurately, which would be overestimated by about 27% if no local calibration is performed on the FAO56 Penman-Monteith model in China. The average ET_o was 769 mm yr⁻¹ based on calibrated radiation model in China during 1971-2000.     

Use of thermal units to estimate corn crop coefficients under semiarid climatic conditions

Two crop coefficient equations were derived as a function of fraction of thermal units from lysimeter measured corn evapotranspiration (ET_c-lys) during 1997 and 1998, and reference evapotranspiration obtained from: (a) lysimeter measurements (Kc_mes) or FAO Penman-Monteith (ET_o-PM) estimates (Kc_est-PM). For validation, corn evapotranspiration (ET_c-est) was estimated in 2005 and 2006 from ET_o-PM and: (a) the equation for Kc_mes with (ET_c-est-lyslc) or without (ET_c-est-lys) locally calibrated ET_o-PM; (b) the equation for Kc_est-PM; and (c) the FAO approach (ET_c-est-FAO). The ET_{c-est_lys} estimates showed the lowest bias (0.09 mm day⁻¹); the ET_c-est-PM and ET_c-est-FAO, the highest (0.50-0.51 mm day⁻¹). However, the root mean square error (RMSE, 1.23–1.27 mm day⁻¹) and the index of agreement (IA, around 0.94) of the ET_c-est-lys, ET_c-est-lyslc and ET_c-est-PM were similar. Therefore, ET_c-est-lys is recommended although the ET_c-est-lyslc was similarly accurate. The ET_c-est-PM is less recommended due to poorer bias and systematic mean square error, and a general underestimation except for low corn ET values. For real time irrigation scheduling, the ET_c-est-FAO should be avoided as RMSE (1.35 mm day⁻¹), IA (0.93) and bias were slightly worse, corn ET was overestimated but for high values, and the length of the four phenological stages must be known in advance.     

An evaluation of common evapotranspiration equations

A comparison is made between the Pruitt and Doorenbos version of an hourly Penman-type equation, the Food and Agriculture Organization (FAO) hourly Penman-Monteith equation, and an independent measure of reference evapotranspiration (ET₀) from lysimeter data. Reducing the canopy resistance improved the hourly FAO Penman-Monteith estimates. Daytime soil heat flux density is estimated as 10% of net radiation in the FAO hourly Penman-Monteith equation; however, the measured soil heat flux density under grass that was never shorter than 0.10 m in this study was between 3% and 5% of net radiation. The daytime totals of hourly ET₀ from the hourly Penman-Monteith and Pruitt-Doorenbos equations and ET₀ from the 24-h FAO Penman-Monteith equation were computed using data from five Italian and five Californian stations. A comparison showed that all of the equations gave acceptable results. The Pruitt-Doorenbos equation may slightly over-estimate ET₀ in conditions of summertime cold air advection. Received: 18 November 1998     

Evapotranspiration and crop coefficients of irrigated biomass sorghum for energy production

New cultivars of sorghum for biomass energy production are currently available. This crop has a positive energy balance being irrigation water the largest energy consumer during the growing cycle. Thence, it is important to know the biomass sorghum water requirements, in order to minimize irrigation losses, thus saving water and energy. The objective of this study was to quantify the water use and crop coefficients of irrigated biomass sorghum without soil water limitations during two growing seasons. A weighing lysimeter located in Albacete (Central Spain) was used to measure the daily biomass sorghum evapotranspiration (ET_c) throughout the growing season under sprinkler irrigation. Seasonal lysimeter ET_c was 721 mm in 2007 and 691 mm in 2010. The 4 % higher ET_c value in 2007 was due to an 8 % higher evaporative demand in that year. Maximum average K _c values of 1.17 in 2007 and 1.21 in 2010 were reached during the mid-season stage. The average K _c values for the 2 years of study were K _c-ini: 0.64 and K _c-mid: 1.19. The seasonal evaporation component was estimated to be about 18 % of ET_c. The average basal K _c (K _cb) values for the two study years were K _cb-ini: 0.11 and K _cb-mid: 1.17. The good linear relationship found between K _cb values and the fraction of ground cover (f _c) and the excellent agreement found between Normalized Difference Vegetation Index and different biophysical parameters, such as K _cb and f _c, will allow monitoring and estimating the spatially distributed water requirements of biomass sorghum at field and regional scales.     

Measured and estimated water use and crop coefficients of grapevines trained to overhead trellis systems in California’s San Joaquin Valley

The use of overhead trellis systems for the production of dry-on-vine (DOV) raisins and table grapes in California is expanding. Studies were conducted from 2006 to 2009 using Thompson Seedless grapevines grown in a weighing lysimeter trained to an overhead arbor trellis and farmed as DOV raisins for the first two years and for use as table grapes thereafter. Maximum canopy coverage for the two lysimeter vines across years was in excess of 80 %. Seasonal (15 March–31 October) evapotranspiration for the lysimeter vines (ET_Lys) was 952 mm in 2007 (farmed as DOV raisins) and 943 and 952 mm (when farmed as table grapes). The maximum crop coefficient (K _cLys) across all 4 years ranged from 1.3 to 1.4. These maximum values were similar to those estimated using the relationship where K _c is a function of the amount of shaded area measured beneath the canopy at solar noon (K _c = 0.017 × percent shaded area). Covering the lysimeter’s soil surface with plastic (and then removing it) numerous times during the 2009 growing season (1 June–14 September) reduced ET_Lys from an average of 6.4 to 5.6 mm day^?1 and the K _c from 1.07 to 0.93. A seasonal basal K _c (K _cb) was calculated for grapevines using an overhead trellis system with a 13 % reduction in the K _cLys across the growing season.     

Using energy balance data for assessing evapotranspiration and crop coefficients in a Mediterranean vineyard

Improving water use efficiency is a key element of water management in irrigated viticulture, especially in arid or semi-arid areas. In this study, the micrometeorological technique “Eddy Covariance” was used to directly quantify the crop evapotranspiration (ET) and to analyze the complex relationships between evapotranspiration, energy fluxes, and meteorological conditions. Both observed Direct measurements (DIR) of latent heat flux (LE) and observed from the residual of the energy balance (REB) equation were used for crop evapotranspiration calculations. Observed crop coefficients (K _cms) were then determined using the standardized reference evapotranspiration (ET_o) equation for short canopies. In addition, linear approximations from observations were used to model the seasonal trend lines for crop coefficients and K _cs values were parameterized by first identifying the beginning and end of each growth stage. The modeled K _cs values were used to predict daily ET from ET_o measurements and compared with values from literature. The daily observed DIR ET values (ET_do) were lower than REB ET (ET_ro) during periods with precipitation, but they were similar during dry periods, which implies that energy balance closure is better when the surface is drier. Comparisons between modeled ET and crop ET estimated using K _c values from best agreement was observed between the modeled REB and FAO 56 and the local K _c values provided by the Regional Agency ARPAS showed good agreement with observed ET (from DIR and REB data) than the FAO 56 ones. The study confirmed that the availability of locally driven K _c could be relevant to quantify the crop water requirement and represents the starting point for a sustainable management of water resources.