Estimating reference evapotranspiration with atmometers in a semiarid environment

Reference evapotranspiration (ET₀) estimations require accurate measurements of meteorological variables (solar radiation, air temperature, wind speed, and relative humidity) which are not available in many countries of the world. Alternative approaches are the use of Class A pan evaporimeters and atmometers, which have several advantages compared to meteorological stations: they are simple, inexpensive and provide a visual interpretation of ET₀. The objectives of the study were to compare the evaporation from atmometers (ET_gage) with the evapotranspiration estimated by the FAO-56 Penman-Monteith equation (ET_0PM) and to evaluate the variability between three modified atmometers of a commercial model. Comparison between daily ET_gage measured by the atmometer and ET_0PM showed a good correlation. However, ET_gage underestimated ET_0PM by approximately 9%. Differences between ET_gage and ET_0PM ranged from −2.4 to 2.2 mm d⁻¹ while the mean bias error was −0.41 mm d⁻¹. Underestimations occurred more frequently on days with low maximum temperatures and high wind speeds. On the contrary, atmometer overestimations occurred on days with high maximum temperatures and low wind speeds. Estimates of ET₀ using the atmometer appeared to be more accurate under non-windy conditions and moderate temperatures as well as under windy conditions and high temperatures. Atmometers 2 and 3 overestimated the evaporated water by atmometer 1 with a maximum variability of cumulative water losses of 4.5%. A temperature-based calibration was performed to improve the atmometer accuracy, using maximum temperature as an independent variable, with good results.     

北京地区潜在蒸散量计算方法的比较研究

利用北京地区1951—2007年的逐日气象资料,选取常用的7种公式计算日潜在蒸散量,并和利用FAO推荐的Penman-Monteith(PM)标准公式计算日潜在蒸散量进行比较。根据线性回归、平方根误差和平均偏差方法分析得出:Penman公式、Kimberly-Penman公式(KP)和Doorenbos-Pruitt公式(DP)与PM相关性较好,KP公式计算的ET0和标准ET0平均偏差和平方根误差均最小,可直接用来计算北京地区的ET0,而Penman公式和DP公式的平均偏差和平方根误差较大,不适合直接计算北京地区的ET0,利用气象数据提出了修正的Penman公式和DP公式。Makkink公式、Priestley-Taylor公式、Hargreaves公式和Turc公式与PM相关性较差,不适合计算北京地区的ET0。北京地区对ET0影响最大的气象因子为饱和水汽压差和净辐射,基于此,提出了2个适合估算北京地区缺资料条件下ET0的经验公式。     

干旱荒漠草原应用Penman-Monteith与Penman修正式计算ET0的偏差分析

甘肃天祝草原位于我国西北干旱荒漠草原,应用天祝县二道墩试验站2005年的实测气象资料,利用Penman-Monteith公式和Penman修正式计算参考作物腾发量(ET0)并进行了比较。Penman修正式计算的参考作物腾发量ET0值略小于Penman-Monteith公式计算的值,最大绝对偏差0.5 mm/d。分析发现生育期辐射项ETrad是导致参考作物腾发量ET0产生偏差的主要原因。2种方法计算的空气动力项ETaero差别较小,最大绝对偏差不超过0.2 mm/d。导致计算偏差的原因在于2种公式采用了不同的辐射项和空气动力学项计算公式和参数。2个公式计算的参考作物腾发量具有显著的线性相关性。     

重庆丘陵山区参考作物蒸散量的确定及气候影响因素分析

基于重庆市北碚区2001—2011年气象资料,采用Penman-Monteith公式计算ET0,分析了3种不同太阳辐射(Rs)计算方法所得ET0的差异性,并利用相关性和敏感性分析方法分析气象因子对ET0的影响。结果表明,研究区2001—2011年年内各月ET0呈抛物线变化,年内ET0最大值出现在7月和8月,最小值出现在1月和12月;不同Rs计算方法是引起ET0和辐射项(ET0(rad))差异的主要原因,但差异不显著;研究区ET0主要由空气动力学项(ET0(aero))贡献;最高温度、最高相对湿度和最低相对湿度是研究区ET0的3个最主要的影响因子;采用Pen-man-Monteith公式计算研究区ET0时,建议采用Hargreaves公式计算Rs。     

考虑地形对气温影响的ET_0计算方法研究

局部地形因子的剧烈变化对气温的影响会进一步影响参考作物需水量(ET0)的精确估算。以贵州省为例开展地形对气温影响下的ET0计算方法,采用符号回归的方法,构建Modis实测气温值与经度、纬度、海拔、坡度、坡向等地形因子以及插值气温之间的函数关系,并将其代入FAO56PM公式,进而得到考虑地形对气温影响的ET0计算方法。研究结果表明,计算所得的气温与Modis气温实测值之间的相关系数均大于0.49,可以有效减小气温的估算误差,具有较好的模拟效果;考虑地形对气温影响的ET0与传统插值方法所得的贵州省内日ET0空间分布特征相一致且较为连续;考虑地形对气温影响下南坡上的ET0大于北坡,且ET0随坡度的增加而减小,当坡度大于60°时,坡度对ET0的影响不容忽视。研究结果可为贵州地区农业精准灌溉和灌区水资源精准配置提供指导依据。     

Peak crop coefficient values for Shaanxi,North-west China

Lysimeter and soil water instruments were used in North-west China to determine peak crop coefficients and calibrate irrigation scheduling software developed for commercial use in Scotland. The (1963) Penman equation, previously used in Scotland, was compared with a particular formulation of the Penman–Monteith (PM) equation proposed by Allen et al. [Allen, R.G., Smith, M., Perrier, A., Pereira, L.S., 1994a. An update for the definition and calculation of reference evapotranspiration. ICID Bull. 43 (2), 1–34; Allen, R.G., Smith, M., Perrier, A., Pereira, L.S., 1994b. An update for the calculation of reference evapotranspiration. ICID Bull. 43 (2), 35–92]. Measures of soil water loss for winter wheat, maize and peaches were compared with scheduling model predictions. Predicted values of cumulative actual water use were comparable for the two ET equations when a soil water stress function was applied. However, the new PM equation provided slightly higher model efficiency, when used with a peak crop coefficient of 1.25 for maize. A peak value of 1.25 also appeared appropriate for wheat, though the evidence was less strong. Inclusion of the slow mobile water concept slightly improved agreement between measured and predicted soil water changes for the maize and peaches. For peaches, the revised PM equation provided minimum values of root mean square error (R.M.S.E.) and bias, when combined with a slow mobile water representation and using a peak crop coefficient of 0.75.     

Alfalfa (Medicago sativa L.) growth response to water and temperature

Summary Four cultivars of alfalfa (Medicago sativa L cv. Vangard, Cody, Zia, and Dawson) were grown under a gradient irrigation system on a Pullman clay loam soil (fine, mixed, thermic, Torrertic Paleustoll) at Bushland, Texas. Twelve harvests were taken during 1983, 1984 and 1985 over a wide range of irrigation levels. Alfalfa was harvested at 10% bloom and subsamples were oven dried and ashed. Irrigation water and rainfall were determined by catchment collections and soil moisture content was determined with a neutron soil moisture probe. Potential evaporation was determined by pan evaporation and by modified Penman, Priestley and Taylor, and Jensen and Haise prediction equations from climatic data. No varietal differences in the relationship of yield to water or water use efficiency occurred. Yield within each harvest correlated well with evapotranspiration (ET). The regression of yield with ET over all twelve harvests had a low coefficient of determination. Relative yield had a high correlation with relative ET when maximum ET was measured but the correlation was lower when maximum ET was calculated from a prediction equation or pan evaporation. The correlation increased when the maximum yield for each harvest was used rather than a constant value. Including a high temperature factor in the equation greatly improved the correlation between yield and ET but the correlation was not as high as when relative yield was correlated with relative measured ET. Water use efficiency was highest with the highest yields.Contribution Texas Agricultural Experiment Station. Paper No. 20929     

关中地区灌溉农业发展对区域蒸发的影响研究

主要从分析参考作物蒸发蒸腾量(ET0)的变化趋势来反映气候变化对蒸发的影响,从灌区实测水面蒸发量的变化趋势来分析灌溉农业发展对区域蒸发的影响。用改进后的Penman公式计算关中地区1961～2001年系列5个气象站的ET0,结果显示:80年代前后气候对关中地区年ET0值的影响有明显区别,80年代以后关中地区年ET0值增长趋势加大,受气候影响明显大于80年代以前。多年平均年内分布表明连续最大3月即6～8月占全年的比例为46%～48%,但80年代以后年内分布6～8月ET0所占比例有降低趋势;从泾惠渠灌区灌溉试验站实测水面蒸发资料分析,显示明显的逐年减少的趋势,年内分布表明6～8月水面蒸发量所占比例有降低趋势。说明灌溉农业发展引起农田小气候的变化,减少了夏季潜在的蒸发和实际蒸发量。     

新疆地区潜在蒸散量计算模型适用性评价

[目的]评价不同潜在蒸散量模型计算结果的准确性以及模型在不同地区的适用性,提出适合于新疆不同区域的潜在蒸散量计算模型.[方法]基于新疆16个站点1970-2015年的实测气象资料,以实测蒸发为基准,采用综合法、辐射法、质量传输法、温度法4类模型计算了潜在蒸散量;通过随机森林模型分析了潜在蒸散量的主要影响因素,并利用多属...     

The accuracy of evapotranspiration estimated with the FAO modified penman equation

Summary The FAO modified Penman equation has gained acceptance as a standard method of estimating reference crop evapotranspiration. Although theoretically sound the Penman equation becomes increasingly empirical when parameters or variables have to be estimated. When evapotranspiration estimates are being used for practical purposes the uncertainties introduced by these empirical factors and relationships should not be neglected. Evapotranspiration estimates for north-east Sri Lanka are used to illustrate the importance of the empiricisms in the FAO modified Penman equation. It is shown that the different empirical relationships used to estimate net radiation and the wind function in the FAO modified Penman equation and in the Penman (1963) equation produce a 23% difference in the estimate of annual reference crop evapotranspiration.     

参考作物需水量计算方法在纵向岭谷区的应用对比

利用纵向岭谷区内58个典型站点1971~2000年逐月气象资料,以及昆明、元江、大理、景洪、保山等5个站点建站至2000年逐日气象资料,从逐日、月及年的不同时间尺度,以Penman Montieth方程计算结果为标准,分析修正Penman法、Priestley Taylor、Hargreaves等不同方法计算参考作物腾发量的适用性。在月和年时段上,修正Penman法较标准值偏小1%~19%,3~10月份平均误差小于6%,各流域之间存在一定差异。不同水文频率年Priestley Taylor和Hargreaves公式计算的逐日ET0,都不同程度地比标准值小,昆明、保山、大理等半干旱或半湿润地区,Priestley Taylor公式计算结果更接近于标准值,平均误差11%~16%左右;景洪等湿润地区Hargreaves公式与标准值误差最小,为15%左右;但接近干旱区的元江则2种方法的结果都存在较大差异,相对误差大于25%;各月ET0的变异系数是Priestley Taylor大于Hargreaves公式,且绝大多数月份小于0.20;不同天气类型时Priestley Taylor计算精度变化大,晴和多云天气情况下的误差小于16.3%,阴雨天则误差比Hargreaves公式大,后者的计算精度在各种天气条件下较稳定;误差在年内的分布是7~8月最小,年初和年末最大,变化趋势与修正Penman法的对比结果相同。各种方法的ET0计算结果与标准值的相关系数均大于0.80。     

春玉米生长中期灌水量差异对其蒸发蒸腾量估算精度的影响

为提高旱区作物蒸发蒸腾量估算精度,以石羊河流域春玉米为研究对象,分析灌水量对FAO-56估算作物蒸发蒸腾量精度的影响,并对估算误差进行讨论,提出使用部分根区含水量平均值用于土壤水分胁迫系数计算.结果表明:FAO-56对不同灌水处理下作物蒸发蒸腾量的估算精度存在较大差异,可较精确地估算低灌水处理下作物蒸发蒸腾量;随着灌水量增大,其估算精度有所降低,对高灌水处理下作物蒸发蒸腾量的估算误差达-14.13%;根区上部土层含水量与土壤水分胁迫状况关系紧密,以缓变层及以上土层含水量平均值代替整个根区含水量平均值用于土壤水分胁迫系数计算,可有效改善高灌水处理下旱区作物蒸发蒸腾量计算精度,亦可较为精确地估算低灌水处理下作物蒸发蒸腾量.     

风沙区参考作物需水量的计算

根据国内外相关的研究成果 ,分析选择并确定了适宜于风沙区参考作物需水量 (ET0 )的计算模式。利用典型风沙区的气象资料 ,对多年逐旬参考作物需水量及 2 0 0 1年春小麦与春玉米生育时段内逐日参考作物需水量进行了分析计算。结果表明 ,FAO最新修正的 Penman-Moteith公式可较好地用于风沙区参考作物需水量的估算 ,一般 ET0 值在年内与年际间变化较大 ,最高值发生在 6月上旬左右 ,多年平均为 5 .82 mm/ d,最低值发生在 1月上旬 ,多年平均 0 .43 mm/ d左右 ,年内各日 ET0 值受气象因素的影响变幅很大 ,因此 ,精确灌溉应设法提高短期天气预报和灌溉预报的精度     

Hargraeves公式计算参考作物腾发量在新疆地区的适用性研究

新疆维吾尔族自治区地域辽阔,气候特征空间差异性显著。准确估算各地区的参考作物腾发量(ET0)是新疆节水灌溉设计的基础。以阿克苏地区30年的气象资料为基础计算了ET0,并以Penman-Monteith公式和修正Penman公式为参考标准,进行对比分析评价Hargraeves公式的精度和地区适应性。结果显示Hargraeves公式计算的参考作物蒸发蒸腾量,精度较Penman公式高,较Penman-Monteith公式低,但满足实际生产精度要求,特别适用与阿克苏地区气候类似的西部地区,基础气象资料不全的地区的参考作物蒸发蒸腾量的计算。     

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.     

Remote sensing to estimate ET-fluxes and the performance of an irrigation district in southern Italy

Satellite remote sensed data on canopy biophysical properties, ground data and agro-meteorological information were combined to estimate evapotranspiration (ET) fluxes of orange orchards using a modified Penman–Monteith equation. The study was carried out during the irrigation season 2004 in an irrigation district, cover for about 95% with orange groves, of 1550 ha located in eastern Sicily (Italy). The spatial pattern in ET-fluxes have been analysed using IKONOS high-resolution satellite and hyper-spectral ground data acquired and processed for the study-area. The remote estimates of ET-fluxes varied between 1.3 and 5.7 mm/day, with a daily average value of about 4.2 mm, showing a good agreement with crop ET values determined as residual of soil water balance of selected ground control sites. Crop coefficient estimates ranged between 0.22 and 1.08 showing positive correlations with percentages of ground cover (C_g) increasing from 30 to 80% ground shading and with LAI values. By comparing ET estimates with water volumes supplied in each sub-district of the study-area, the performance indicator “IP” was evaluated, allowing to rank the conditions of un-fulfilment of crop water requirements by public and private water distribution systems. Generally, out of 29 sub-districts, 14 had “IP” values less than 50%, revealing a sub-optimal water supply for the study-area.     

柳园口灌区作物ET_0变化及节水灌溉模式研究

为了研究黄河下游灌区作物需水量变化规律,以柳园口灌区为例,采用国际上通用的Penman公式,分析研究主要参考作物的需水量变化情况,并结合引黄河水流量与含沙量的变化特点,探讨了灌区节水灌溉模式。研究表明:柳园口灌区ET_0呈近年来处于稳定态势,整体略有下降,主要农作物受生长季节和生育周期的限制,棉花生育期需水量大于小麦,小麦的需水量大于玉米。结合黄河来水来沙分布情况,引导灌区适时使用引黄灌溉和井灌的灌溉模式,充分发挥有限水资源的最大效益。     

A statistical approach for estimating irrigation water usage in western Saudi Arabia

This paper presents a statistical practical approach to the estimation of irrigation water requirement in the western region of Saudi Arabia. The approach describes the variation in water requirements as a function of regional climatic parameters, types of crop grown and irrigation techniques used. Irrigation water requirements are also calculated, for comparison purposes, by Modified Penman and Blaney-Criddle methods, taking leaching requirements and losses into consideration. It is found that irrigation water requirements for vegetables and perennial crops are close to the values estimated by Blaney-Criddle method, while for fodder and cereal they are close to the values estimated by Modified Penman Method.     

The effect of water stress and rootstock on the development of leaf injuries in grapevines irrigated with saline effluent

Deficit irrigation is increasingly being practiced in water-limited areas to overcome water scarcities. Although, this practice reduces yield losses, there is limited information currently available on how this practice can affect crops when the irrigation water contains elevated levels of salts. An experiment was set up to investigate salt uptake and distribution and salt tolerance of potted Soultanina vines grafted on different rootstocks (41B, 1103P, 110R) and irrigated with effluent containing relatively high concentrations of salts and fresh water at different fractions of evapotranspiration (0.50, 0.75 and 1.00ET). Irrigation with recycled water induced the development of leaf burns, which were more intense in 1998 despite the lower leaf-Na and -Cl content. This may have been due to the more severe water deficit and/or to the more adverse climatic conditions which prevailed during that season. Decreasing the irrigation level induced the development of leaf burns causing only minor changes to leaf-Na or -Cl content. Differences in salt uptake, accumulation and distribution were observed among the rootstocks investigated in this work, suggesting that differences exist in the mechanisms regulating salt uptake and distribution in the shoot. Despite these differences, a distinct superiority in terms of salinity tolerance among rootstocks was only observed at the 0.50ET irrigation level, where vines grafted on 41B developed earlier and more acute leaf burns than the other rootstocks. These findings suggest that leaf salt content alone it is not to classify genotypes according to their tolerance to salinity and that salinity-induced damage is linked with prevailing environmental conditions. Furthermore, it can be inferred that grapevines have additional mechanisms to cope with salt stress which may counteract differences in salt uptake and accumulation in the shoot.     

Minimizing instrumentation requirement for estimating crop water stress index and transpiration of maize

Research was conducted in northern Colorado in 2011 to estimate the crop water stress index (CWSI) and actual transpiration (T _a) of maize under a range of irrigation regimes. The main goal was to obtain these parameters with minimum instrumentation and measurements. The results confirmed that empirical baselines required for CWSI calculation are transferable within regions with similar climatic conditions, eliminating the need to develop them for each irrigation scheme. This means that maize CWSI can be determined using only two instruments: an infrared thermometer and an air temperature/relative humidity sensor. Reference evapotranspiration data obtained from a modified atmometer were similar to those estimated at a standard weather station, suggesting that maize T _a can be calculated based on CWSI and by adding one additional instrument: a modified atmometer. Estimated CWSI during four hourly periods centered on solar noon was largest during the 2 h after solar noon. Hence, this time window is recommended for once-a-day data acquisition if the goal is to capture maximum stress level. Maize T _a based on CWSI during the first hourly period (10:00–11:00) was closest to T _a estimates from a widely used crop coefficient model. Thus, this time window is recommended if the goal is to monitor maize water use. Average CWSI over the 2 h after solar noon and during the study period (early August to late September, 2011) was 0.19, 0.57, and 0.20 for plots under full, low-frequency deficit, and high-frequency deficit irrigation regimes, respectively. During the same period (50 days), total maize T _a based on the 10:00–11:00 CWSI was 218, 141, and 208 mm for the same treatments, respectively. These values were within 3 % of the results of the crop coefficient approach.