Evaluation of pan coefficient for reference crop evapotranspiration for semi-arid region

Pan coefficient (K _pan) is the important factor for computation of reference evapotranspiration (ET _o ) from pan evaporation (E _pan). In this paper, the approaches proposed by Cuenca (Irrigation system design: an engineering approach. Prentice-Hall, Englewood Cliffs, 1989), Snyder (J Irrig Drain Eng 118(6):977–980, 1992), Orang (Potential accuracy of the popular non-linear regression equations for estimating pan coefficient values in the original and FAO-24 tables. Unpublished Report, Calif. Dept. of Water Resources, Sacramento, 1998), Raghuwanshi and Wallender (J Irrig Drain Eng 118 (6):977–980, 1998) and Pereira et al. (Agric Water Manage 76:75–82, 1995) were evaluated for a semi-arid region. By comparing with the FAO 56 Penman-Monteith (F-PM) method the Snyder (J Irrig Drain Eng 118(6):977–980, 1992, 1992) approach was best suited for the semi-arid region.     

Review of measured crop water productivity values for irrigated wheat,rice, cotton and maize

The great challenge of the agricultural sector is to produce more food from less water, which can be achieved by increasing Crop Water Productivity (CWP). Based on a review of 84 literature sources with results of experiments not older than 25 years, it was found that the ranges of CWP of wheat, rice, cotton and maize exceed in all cases those reported by FAO earlier. Globally measured average CWP values per unit water depletion are 1.09, 1.09, 0.65, 0.23 and 1.80 kg m⁻³ for wheat, rice, cotton_seed, cotton_lint and maize, respectively. The range of CWP is very large (wheat, 0.6–1.7 kg m⁻³; rice, 0.6–1.6 kg m⁻³; cotton_seed, 0.41–0.95 kg m⁻³; cotton_lint, 0.14–0.33 kg m⁻³ and maize, 1.1–2.7 kg m⁻³) and thus offers tremendous opportunities for maintaining or increasing agricultural production with 20–40% less water resources. The variability of CWP can be ascribed to: (i) climate; (ii) irrigation water management and (iii) soil (nutrient) management, among others. The vapour pressure deficit is inversely related to CWP. Vapour pressure deficit decreases with latitude, and thus favourable areas for water wise irrigated agriculture are located at the higher latitudes. The most outstanding conclusion is that CWP can be increased significantly if irrigation is reduced and crop water deficit is intendently induced.     

Evapotranspiration and crop coefficient of spring maize with plastic mulch using eddy covariance in northwest China

Spring maize under plastic mulch is the staple food crop in northwest China. Studying its evapotranspiration (ET) and crop coefficient (K_c) is important for managing water-saving irrigation in the region. Eddy covariance (EC) was applied to measure spring maize ET in 2007 in northwest China, focusing on the characteristics of the maize ET and K_c processes under plastic mulch. An interesting result was that a higher K_c in this study relative to the value of FAO 56 was presented in the mid and late season, e.g. average K_c was 1.46, 1.39 and 1.22 during the heading, filling and maturity stage, respectively. This result was mainly due to that (1) the plastic mulch had an effect on anti-senescence of maize and great green leaf still existed before the harvest; (2) the FAO 56 PM model may underestimate the reference crop ET in the mid and late season of maize in the region; (3) the planting density was higher in the study, which was about 374,800 plants ha⁻¹. Though K_c during the mid and late season was high, a high water use efficiency of 25.2 kg ha⁻¹ mm⁻¹ was still obtained in the study. Our study confirmed that plastic mulch has beneficial effect on improving maize water use efficiency in this severe water shortage region of northwest China.     

Cover crop evapotranspiration under semi-arid conditions using FAO dual crop coefficient method with water stress compensation

Cover cropping is a common agro-environmental tool for soil and groundwater protection. In water limited environments, knowledge about additional water extraction by cover crop plants compared to a bare soil is required for a sustainable management strategy. Estimates obtained by the FAO dual crop coefficient method, compared to water balance-based data of actual evapotranspiration, were used to assess the risk of soil water depletion by four cover crop species (phacelia, hairy vetch, rye, mustard) compared to a fallow control. A water stress compensation function was developed for this model to account for additional water uptake from deeper soil layers under dry conditions. The average deviation of modelled cumulative evapotranspiration from the measured values was 1.4% under wet conditions in 2004 and 6.7% under dry conditions in 2005. Water stress compensation was suggested for rye and mustard, improving substantially the model estimates. Dry conditions during full cover crop growth resulted in water losses exceeding fallow by a maximum of +15.8% for rye, while no substantially higher water losses to the atmosphere were found in case of evenly distributed rainfall during the plant vegetation period with evaporation and transpiration concentrated in the upper soil layer. Generally the potential of cover crop induced water storage depletion was limited due to the low evaporative demand when plants achieved maximum growth. These results in a transpiration efficiency being highest for phacelia (5.1 g m⁻² mm⁻¹) and vetch (5.4 g m⁻² mm⁻¹) and substantially lower for rye (2.9 g m⁻² mm⁻¹) and mustard (2.8 g m⁻² mm⁻¹). Taking into account total evapotranspiration losses, mustard performed substantially better. The integration of stress compensation into the FAO crop coefficient approach provided reliable estimates of water losses under dry conditions. Cover crop species reducing the high evaporation potential from a bare soil surface in late summer by a fast canopy coverage during early development stages were considered most suitable in a sustainable cover crop management for water limited environments.     

The influence of crop canopy on evapotranspiration and crop coefficient of beans (Phaseolus vulgaris L.)

Experiments were conducted to investigate the effect of crop development on evapotranspiration and yield of beans (Phaseolus vulgaris L.) at the Instituto Agronômico (IAC), Campinas, State of São Paulo, Brazil, during the dry season of 1994. A completely randomized design was carried out with three population density treatments and four replications. The treatments were: (a) crop sown in evapotranspirometers at a density of 50 plants m⁻², and thereafter thinned to 25 plants m⁻², when the canopy achieved full ground cover; (b) crop sown with population densities of 14 and 28 plants m⁻² in an irrigated field. Crop growth was evaluated considering dry matter (DM), vegetative ground cover (GC%) and leaf area index (LAI). These parameters were successfully related to basal crop coefficient (k_cb) and crop coefficient (k_c), demonstrating the strong dependence of both coefficients on canopy development. A simulation study was carried out and showed that k_cb based on LAI would allow good estimates of water use for different plant density populations in the field.     

西北地区农业结构与水资源的相关分析

农业内部不同种植方式对水资源的消耗不同,农业结构与农业用水结构间的相关度也不一样.为促进西北地区有限水资源可持续利用与农业可持续发展,在运用相关分析理论和模型揭示农业结构与农业用水结构之间相互关系的基础上,探讨通过农业结构调整优化农业用水结构的可能性,得出西北地区应降低粮食作物种植比重,发展经济作物与人工草业的结论.     

A consolidated evaluation of the FAO-56 dual crop coefficient approach using the lysimeter data in the North China Plain

The main purpose of this paper was to evaluate whether or not the dual crop coefficient (DCC) method proposed in FAO-56 was suitable for calculating the actual daily evapotranspiration of the main crops (winter wheat and summer maize) in the North China Plain (NCP). The results were evaluated with the data measured by the large-scale weighing lysimeter at the Yucheng Comprehensive Experimental Station (YCES) of the Chinese Academy of Sciences (CAS) from 1998 to 2005 using the Nash-Sutcliffe efficiency (NSE), the root mean square error (RMSE) and the root mean square error to observations’ standard deviation ratio (RSR). The evaluation results showed that the DCC method performed effective in simulating the quantity of seasonal evapotranspiration for winter wheat but was inaccurate in calculating the peak values. The RMSE value of the winter wheat during the total growing season was less than 0.9 mm/d, the NSE and RSR values during the total growing stage were “Very Good”, but the results for summer maize were “Unsatisfactory”. The recommended basal crop coefficient values K_cbtab during the initial, mid-season and end stages for winter wheat and summer maize were modified and the variation scope of basal crop coefficient K_cb was analyzed. The K_c (compositive crop coefficient, K_c = ET_c/ET₀, ET_c here is the observed values by lysimeter, ET₀ is the reference evapotranspiration) values were estimated using observed weighing lysimeter data during the corresponding stages for winter wheat and summer maize were 0.80, 1.15, 1.25, 0.95; 0.90, 0.95, 1.25, 1.00, respectively. These can be a reference for irrigation planning.     

Determination of evapotranspiration and basal crop coefficient of alfalfa with a weighing lysimeter

The aim of this study was to determine the evapotranspiration and basal crop coefficient of alfalfa with a weighing lysimeter. Lysimeter experiments were conducted at Ankara Research Institute of Rural Services, Turkey during 1995–1997. Penman–Monteith, Makkink (FAO-radiation (FAO-Rad)) and Penman-FAO methods performed satisfactorily for estimating the reference evapotranspiration of alfalfa in semi-arid climate conditions. Measured evapotranspiration rates were 1470, 1557 and 1161 mm in 1995, 1996 and 1997, respectively. Basal crop coefficients were computed from evapotranspiration measurements and weather data. The estimated values of the basal crop coefficients for alfalfa at the four growth stages of an individual cutting period were 0.71, 1.78 and 1.51 for initial, mid and late season, respectively.     

Evapotranspiration and crop coefficient of Populus euphratica Oliv forest during the growing season in the extreme arid region northwest China

Based on successive observation, fifteen-day evapotranspiration (ET_c) of Populus euphratica Oliv forest, in the extreme arid region northwest China, was estimated by application of Bowen ratio-energy balance method (BREB) during the growing season in 2005. During the growing season in 2005, total ET_c was 446.96 mm. From the beginning of growing season, the ET_c increased gradually, and reached its maximum value of 6.724 mm d⁻¹ in the last fifteen days of June. Hereafter the ET_c dropped rapidly, and reached its minimum value of 1.215 mm d⁻¹ at the end of growing season. The variation pattern of crop coefficient (K_c) was similar to that of ET_c. From the beginning of growing season, the K_c value increased rapidly, and reached its maximum value of 0.623 in the last fifteen days of June. Afterward, with slowing growth of P. euphratica, the value dropped rapidly to the end of growing season. According to this study, the ET_c of P. euphratica forest is affected not only by meteorological factors, but by water content in soil.     

基于ELM的西北旱区参考作物蒸散量预报模型

为实现气象资料缺失情况下ET₀的精确预报,选取中国西北旱区4个代表性站点的气象资料,建立15种基于极限学习机(ELM)的ET₀预报模型,并通过与其他ET₀计算模型对比和可移植性分析探究ELM在西北旱区的适用性.结果表明:基于温度和风速的ELM₇预报精度较高(整体评价指标GPI排名第4);基于温度和辐射的ELM₅预报精度(GPI排名第6)明显高于Iramk模型和Jensen-Haise模型;仅基于温度的ELM₉预报精度(GPI排名第8)高于Hargreaves-Samani模型.通过模型可移植性分析发现,ELM₇在西北旱区内各训练站点和预测站点组合下预报精度良好.因此,可将ELM₅(输入温度和辐射)、ELM₇(输入温度和风速)和ELM₉(输入温度)作为西北旱区较少气象参数输入情况下精确预报ET₀的推荐模型.     

Evapotranspiration, crop coefficient and growth of two young pomegranate (Punica granatum L.) varieties under salt stress

Pomegranate (Punica granatum L.) is a drought-hardy crop, suited to arid and semi-arid regions, where the use of marginal water for agriculture is on the rise. The use of saline water in irrigation affects various biochemical processes. For a number of crops, yields have been shown to decrease linearly with evapotranspiration (ET) when grown in salt-stressed environments. In the case of pomegranate, little research has been conducted regarding the effect of salt stress. Our study focused on the responses of ET, crop coefficient (K_c) and growth in pomegranate irrigated with saline water. Experiments were conducted using lysimeters with two varieties of pomegranate, P. granatum L. vars. Wonderful and SP-2. The plants were grown with irrigation water having an electrical conductivity (EC_iw) of 0.8, 1.4, 3.3, 4.8 and 8 dS m⁻¹. Plants were irrigated with 120% of average lysimeter-measured ET. Seasonal variation in ET, crop coefficient (K_c) and growth were recorded. Variation in daily ET was observed 1 month after initiation of the treatments. While significant seasonal ET variation was observed for the EC-0.8 treatment, it remained more stable for the EC-8 treatment. Salinity treatment had a significant effect on both daily ET (F = 131, p < 0.01) and total ET (F = 112.68, p = 0.001). Furthermore, the electrical conductivity of the drainage water (EC_dw) in the EC-8 treatment was five times higher than that of the EC-0.8 treatment in the peak season. Fitting the relative ET (ET_r) to the Maas and Hoffman salinity yield response function showed a 10% decrease in ET per unit increase in electrical conductivity of the saturated paste extract (EC_e) with a threshold of 1 dS m⁻¹. If these parameters hold true in the case of mature pomegranate trees, the pomegranate should be listed as a moderately sensitive crop rather than a moderately tolerant one. Fitting 30-day interval ET_r data to the Maas and Hoffman salinity yield response function showed a reduction in the slope as the season progressed. Thus using a constant slope in various models is questionable when studying crop-salinity interactions. In addition, both of the varieties showed similar responses under salt stress. Moreover, the calculated value of K_c is applicable for irrigation scheduling in young pomegranate orchards using irrigation water with various salinities.     

Integrated decision making for reservoir,irrigation, and crop management

An integrated approach to reservoir, irrigation, and cropping management which links four different models—a hydrologic model (PRMS), a crop growth simulation model (EPIC), an economic model based on linear programming, and a dynamic programming model—is developed and demonstrated. The demonstration is based on an irrigation district located in a subhumid climate with an irrigation reservoir large enough for over-year storage. The model is used to make repeated simulations for various planning horizons. Two different types of results are presented. The first provides the probability that each of the various farm plans (land/crop/water allocation) will be chosen as the optimum in the first year of the planning horizon. The second approach provides probability distributions of accumulated revenues over a chosen length of planning horizon. Each distribution is associated with an initial reservoir level and a particular farm plan in the first year of the planning horizon. The consequence of selecting certain farm plans at the beginning of a specified planning horizon is therefore quantified in a probabilistic way. Based on families of probability–revenue curves, an irrigation manager can simultaneously evaluate crop, irrigation, and reservoir management options.     

Water management and crop production for food security in China: A review

Food security is a high priority issue on the Chinese political agenda. China’s food security is challenged by several anthropogenic, sociopolitical and policy factors, including: population growth; urbanization and industrialization; land use changes and water scarcity; income growth and nutritional transition; and turbulence in global energy and food markets. Sustained growth in agricultural productivity and stable relations with global food suppliers are the twin anchors of food security. Shortfalls in domestic food production can take their toll on international food markets. Turbulence in global energy markets can affect food prices and supply costs, affecting food security and poverty. Policy safeguards are needed to shield food supply against such forces. China must make unremitting policy responses to address the loss of its fertile land for true progress towards the goal of national food security, by investing in infrastructure such as irrigation, drainage, storage, transport, and agricultural research and institutional reforms such as tenure security and land market liberalization. The links between water and other development-related sectors such as population, energy, food, and environment, and the interactions among them require reckoning, as they together will determine future food security and poverty reduction in China. Climate change is creating a new level of uncertainty in water governance, requiring accelerated research to avoid water-related stresses.     

Model validation and crop coefficients for irrigation scheduling in the North China plain

ISAREG is a model for simulation and evaluation of irrigation scheduling. The model performs the soil water balance and evaluates impacts of water stress on yields for different crops. It is now being used to support a water saving irrigation scheduling program in a pilot area in the North China plain. This paper reports on the calibration and validation of the model using independent data sets relative to winter wheat and summer maize. Data are originated from the Wangdu experimental station and concern a set of drainage lysimeters where diverse irrigation treatments were applied representing different strategies of deficit irrigation. The calibration of the model was performed by deriving the crop coefficients adapted to the local climatic conditions, and considering the soil freezing during winter. The validation of the model was performed using different data sets. Results show that the relative errors to estimate the soil water content averaged 5.3% for summer maize and 7.3% for the winter wheat. These results support the use of the model in the practice.     

Using remote sensing to evaluate the spatial variability of evapotranspiration and crop coefficient in the lower Rio Grande Valley,New Mexico

Pecan is a major crop in the lower Rio Grande Valley (LRGV), New Mexico. Currently, about 11,000 ha of pecan orchards at various stages of growth are consuming about 40% of irrigation water in the area. Pecan evapotranspiration (ET) varies with age, canopy cover, soil type and method of water management. There is a need for better quantification of pecan ET for the purpose of water rights adjudication, watershed management and agronomical practices. This paper describes a process where remote sensing information from Landsat-5 and Landsat-7 were combined with ground level measurements to estimate pecan ET and field scale actual crop coefficient (K _c) for the LRGV. The results showed that annual pecan water use for 279 fields ranged from 498 to 1,259 mm with an average water use of 1,054 mm. For fields with NDVI > 0.6 (normalized difference vegetation index), which represented mature orchards (total of 232 fields), the annual water use ranged from 771 to 1,259 mm with an average water use of 1,077 mm. The results from remote sensing model compared reasonably well with ground level ET values determined by an eddy covariance system in a mature pecan orchard with an average error of 4% and the standard error of estimate (SEE) ranging from 0.91 to 1.06 mm/day. A small fraction (5%) of the pecan fields were within the range of maximum ET and K _c.     

参考作物蒸发蒸腾量计算方法在海河流域的适用性

参考作物蒸发蒸腾量（ET0）的计算公式很多,各公式所需参数各异,为寻找一种所需资料少而又精度较高的替代方法,选用1998年FAO-56分册推荐的Penman－Monteith（PM）、Hargreaves、Irmark-Allen等6种方法分别计算海河流域10个典型气象站30 a的参考作物蒸发蒸腾量,并以PM公式为标准,对其他方法进行评价。结果表明,10个站点中除了五台山地区,Hargreaves与FAO-24 Radiation 这2种方法更接近于PM方法的计算结果,其误差较小,在海河流域缺少辐射和风速     

Modeling the role of irrigation in winter wheat yield,crop water productivity,and production in China

Irrigation plays an important role in increasing food production in China. The impact of irrigation on crop yield (Y), crop water productivity (CWP), and production has not been quantified systematically across regions covering the whole country. In this study, a GIS-based EPIC model (GEPIC) was applied to simulate Y and CWP for winter wheat (Triticum aestivum L.) in China at a grid resolution of 5 arc-minutes and to analyze the impacts of reducing irrigation water on wheat production. The findings show that irrigation is especially important in improving CWP of winter wheat in the North China Plain (NCP), the “bread basket” of China. On average, the provincial aggregate CWP was 56% higher under the irrigated than that under the rainfed conditions. The intensification of water stress and the associated increase in environmental problems in much of the NCP require critical thoughts about reducing water allocation for irrigated winter wheat. Two scenarios for irrigation reduction in the NCP provinces are presented: reducing irrigation depth (S1), and replacing irrigated winter wheat by rainfed winter wheat (S2). The simulation results show that S1 and S2 have similar effects on wheat production when the reduction in irrigation water supply is below 20% of the current level. Above this percentage, S2 appears to be a better scenario since it leads to less reduction in wheat production with the same amount of water saving.     

辽宁省西北部地区旱田增墒技术研究

辽宁省西北部地区水资源紧缺,春旱、秋吊严重,农业生态环境十分脆弱.通过实施增施有机肥、机械深松深翻营造土壤水库技术和补水增墒--机械化一条龙抗旱坐水种技术,增加旱田蓄水能力、提高水分利用效率,可促进辽宁省西北部地区旱地作物高产、优质、高效地发展.     

中国灌溉水粮食生产率及其时空变异

为了评价农业引水总量的生产效益,从全国459个主要灌区的实际灌溉水和粮食生产数据入手,计算、比较了1998,2005和2010年31个省区的灌溉水粮食生产率,利用空间自相关分析方法对1998—2010年中国灌溉水粮食生产率的时空变异规律进行探究.结果显示：各省区灌溉水粮食生产率呈增大趋势,代表年的中国均值为1.03 kg/m3,最大、最小值分别为河南的2.15kg/m3和海南的0.25 kg/m3,区域间差异较大;灌溉水粮食生产率在空间上存在显著的聚集现象,聚集程度随时间变化不明显,高值省区以黄淮海平原为核心集中分布,长江以南则密集了低值省区;江西、安徽及重庆灌溉水粮食生产率的变化幅度与其相邻省区不同步,造成了局部分异特征的变化.分析了中国灌溉水粮食生产率格局的形成及其随时间变化的原因.     

Rating land for crop introduction

A land rating methodology for crop introduction was developed to predict yield and inputs needed to sustain the predicted yield with minimal field experimentation. The methodology was designed to achieve dependable crop introduction without resorting to many long-term expensive field experiments. Yield-affecting variables were integrated into a few land qualities for the convenience of comparing the suitability of different sites in terms of growing the crop. Most of the land qualities of different sites, such as nutrition, were brought up to equal the inputs requirement estimation. Comparisons between sites with these land qualities were made unnecessary. This methodology was built on existing yield, characteristics of the land where yield data are available, Soil Taxonomy, weather data and the growth modelling methodology.