Estimating evapotranspiration from midday canopy temperature

Summary Measurements were made at New Delhi (India) on wheat (Triticum aestivum L.) and mungbean (Vigna radiata L.) crops growing during the winter and summer seasons respectively, to evaluate the use of the equation of Bartholic, Namken and Wiegand (1970) for estimating daytime evapotranspiration from a single measured canopy temperature. Measurements on eleven days indicated that a single observation of the canopy temperature taken at any time between noon and 2 p.m. could be used to compute daytime evapotranspiration with an error (underestimate) of less than 27% as compared with values measured by the Bowen ratio method. The regression equation ET₀ = 0.618 + 0.96 ET₁ has a coefficient of determination of 0.927 and can be used to relate daytime evapotranspiration by the Bowen ratio method (ET₀) to that estimated by the Bartholic-Namken-Wiegand equation (ET₁) for wheat and mung bean crops grown under adequately watered conditions in the New Delhi region.     

Crop coefficients and actual evapotranspiration of a drip-irrigated Merlot vineyard using multispectral satellite images

A field experiment was carried out to evaluate the METRIC (mapping evapotranspiration at high resolution with internalized calibration) model to estimate the actual evapotranspiration (ET_a) and crop coefficient (K _c) of a drip-irrigated Merlot vineyard during the 2007/2008 and 2008/2009 growing seasons. The Merlot vineyard located in the Talca Valley (Chile) was trained on a vertical shoot positioned system. The performance of METRIC was evaluated using measurements of ET_a and K _c from an eddy covariance (EC) system. METRIC overestimated ET_a by about 9?% with a root mean square error (RMSE) and mean absolute error (MAE) of 0.62 and 0.50?mm?d^?1, respectively. For the main phenological stages of the Merlot vineyard, METRIC overestimated the K _c by about 10?% with RMSE?=?0.10 and MAE?=?0.08. Furthermore, the indexes of agreement were 0.70 for K _c and 0.85 for ET_a. Mean values of K _c measured from EC were 0.41, 0.53, 0.56, and 0.46, while those estimated by METRIC were 0.46, 0.54, 0.59, and 0.62 for the bud break to flowering, flowering to fruit set, fruit set to veraison, and veraison to harvest stages, respectively.     

Partitioning of seasonal evapotranspiration from a commercial furrow-irrigated Sultana vineyard

Seasonal partitioning of evapotranspiration (ET) between transpiration by grapevines (Vitis vinifera) (T _gp) and by cover crops of a ryegrass/clover mixture (T _cc), and soil evaporation (E _s) was performed for a furrow-irrigated vineyard during the 1994/1995 and 1995/1996 growing seasons in south-eastern Australia. ET, determined with a water balance approach, averaged 622 mm. The ET rate averaged over the two seasons increased from 2 mm day^–1 in spring (September to November), when it was dominated by T _cc, to peak rates of around 5 mm d^–1 in summer (December to February) when it was dominated by E _s. T _gp, determined with either heat-pulse sensors or the Penman-Monteith equation, attained peak rates of 0.75 and 0.98 mm d^–1, or 6.2 and 8.1 l vine^–1 day^–1 in the first and second seasons, respectively. Total seasonal T _gp of 109.1 mm (900 l vine^–1) in 1994/1995 and 118.8 mm (980 l vine^–1) in 1995/1996 constituted just 18 – 19% of total ET. T _cc totalled 214 mm (34% of ET) in the first season, when pasture cover was sparse and present for 5 months of the growing season (September to February), and 196 mm (30% of ET) in the second season when pasture cover was heavy but present for only 3 months (September to November). E _s averaged 49% of total ET over both seasons. At least 30% of water used for ET was contributed by antecedent soil water in both seasons. The crop factor (K _c) was largely constant throughout the season with an average value of 0.48. The depletion pattern of soil water indicated that the vine explored the soil profile well beyond 1.0 mm depth and almost evenly up to a distance of 1.5 m from the trunk. Water use efficiencies for fresh fruit yield (WUE), i. e., the ratio of fruit weight to total water use at harvest,were 13.3 and 40.5 kg ha^–1 mm^–1 when based on ET in 1994/1995 and 1995/1996, respectively, and 84.0 and 211.1 kg ha^–1 mm^–1, respectively, when based on T _gp. The T _gp data were used to verify three models of vine transpiration developed in an earlier study. Models based on the green area index or on fraction of incident radiation intercepted by the vine canopy produced good agreement with T _gp. The model based on canopy resistance performed poorly, indicating the difficulty of extrapolating the stomatal response to environmental variables from one set of experimental conditions to another. Received: 23 September 1996     

Estimating evapotranspiration in the Padova Botanical Garden

In 1996, intensive building operations near the botanical garden of Padova, the oldest botanical garden in the world, altered a long-established equilibrium between groundwater and plants and threatened the lives of some of them. To avoid water stress, an advanced irrigation system was installed. For design purposes, better knowledge of the water cycle and the monthly average evapotranspiration (ET) in the area was needed. Due to the complex canopy stand of the site, ET was estimated using the water balance method, integrating mathematical models with the Arc/Info Geographical Information System. The water balance was estimated in 1997 and 1998, and results were used to derive an empirical mean crop coefficient of the botanical garden, to simulate the long-term water requirements using the product of reference ET and the apparent crop coefficient to estimate ET from the garden. Two types of hydrological behaviour were identified: one in the central area of the garden, where reduced ground cover diminishes ET and increases runoff and percolation. In the external area, the ET was higher because of the presence of many trees. The empirical mean monthly crop coefficient ranged between 0.56 and 0.83, indicating that ET in the entire area is always less than grass reference ET.     

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.     

Estimating actual irrigation application by remotely sensed evapotranspiration observations

Water managers and policy makers need accurate estimates of real (actual) irrigation applications for effective monitoring of irrigation and efficient irrigation management. However, this information is not readily available at field level for larger irrigation areas. An innovative inverse modeling approach was tested for a field in an irrigation scheme in southern Spain where observed actual evapotranspiration by satellites was used to assess irrigation application amounts. The actual evapotranspiration was used as the basis for an optimization procedure using the physical based SWAP model and the parameter optimization tool PEST. To evaluate the proposed techniques two steps were taken. First, actual observed evapotranspiration from remote sensing was used to optimize two parameters of the SWAP model to determine irrigation applications. Second, a forward-backward approach was applied to test the minimum overpass return time of satellites and the required accuracy of remotely sensed actual evapotranspiration for accurate assessment of irrigation applications. Results indicate that irrigation application amounts can be estimated reasonably accurately, providing data are available at an interval of 15 days or shorter and the accuracy of the signal is 90% or higher.     

Estimating cotton evapotranspiration crop coefficients with a multispectral vegetation index

Crop coefficients are a widely used and universally accepted method for estimating the crop evapotranspiration (ET_c) component in irrigation scheduling programs. However, uncertainties of generalized basal crop coefficient (K_cb) curves can contribute to ET_c estimates that are substantially different from actual ET_c. Limited research with corn has shown improvements to irrigation scheduling due to better water-use estimation and more appropriate timing of irrigations when K_cb estimates derived from remotely sensed multispectral vegetation indices (VIs) were incorporated into irrigation-scheduling algorithms. The purpose of this article was to develop and evaluate a K_cb estimation model based on observations of the normalized difference vegetation index (NDVI) for a full-season cotton grown in the desert southwestern USA. The K_cb data used in developing the relationship with NDVI were derived from back-calculations of the FAO-56 dual crop coefficient procedures using field data obtained during two cotton experiments conducted during 1990 and 1991 at a site in central Arizona. The estimation model consisted of two regression relations: a linear function of K_cb versus NDVI (r²=0.97, n=68) used to estimate K_cb from early vegetative growth to effective full cover, and a multiple regression of K_cb as a function of NDVI and cumulative growing-degree-days (GDD) (r²=0.82, n=64) used to estimate K_cb after effective full cover was attained. The NDVI for cotton at effective full cover was ~0.80; this value was used to mark the point at which the model transferred from the linear to the multiple regression function. An initial evaluation of the performance of the model was made by incorporating K_cb estimates, based on NDVI measurements and the developed regression functions, within the FAO-56 dual procedures and comparing the estimated ET_c with field observations from two cotton plots collected during an experiment in central Arizona in 1998. Preliminary results indicate that the ET_c based on the NDVI-K_cb model provided close estimates of actual ET_c.Communicated by R. Evans     

Monitoring scale-specific and temporal variation in electromagnetic conductivity images

In semiarid and arid landscapes, irrigation sustains agricultural activity but because of increasing demands on water resources there is a need to make gains in efficiency. As such spatial variation of soil properties such as clay and salinity needs to be understood because they strongly influence soil moisture availability. One way is to use electromagnetic induction because apparent soil electrical conductivity (EC_a) is related to volumetric soil moisture (θ), clay and salinity (EC_e). However, depth-specific variation has not been explored. Our aim is to generate electromagnetic conductivity images (EMCIs) by inverting DUALEM-421 EC_a and show how true electrical conductivity (σ) can be correlated with θ, clay, EC_e and bulk density (ρ) on different days post-irrigation (i.e., 1, 4 and 12 days). Two-dimensional multi-resolution analysis (MRA) is used to show how spatio-temporal variation in σ is scale-specific and how soil properties influence σ at different scales. We study this beneath a pivot irrigated alfalfa crop. We found that σ on days 1 and 4 was correlated with θ (Pearson’s r = 0.79 and 0.61) and clay (0.86 and 0.80) and the dominant scale of variation occurred at 9.3–18.7 m (50.21 % of total variation), >74.7 m (23.18 %) and 4.7–9.3 m (16.29 %). Between 9.3–18.7 and 4.7–9.3 m the variation may be a function of the cutter width (8 m), while >74.7 m may be change in clay and EC_e and gantry spacing (~48 m). The sprinkler spacing (1.2 and 1.6 m) explains short-scale variation at 1.2–2.3 m.     

Growing season evapotranspiration from duplex soils in south-western Australia

In the Mediterranean-type climate of south-western Australia, evapotranspiration (ET) is of particular importance because of the expanding threat of dryland salinity. This paper reports on studies at two sites for a total of 5 years aimed at quantifying ET from pastures on duplex (sand over clay) soils within the region. ET was assessed with the Bowen ratio energy balance technique, and compared with potential ET and ET calculated from the water balance. ET closely matched potential ET from the break of the season (May or June) until some seasonally dependent time in spring (September or October). The date of divergence of actual from potential ET was not clearly defined, but occurred gradually during a 2-week period, and was associated with the rapid depletion of soil water at the end of the season. The gradual transition was due to natural variation in the magnitude of daily potential ET during the transition period, which ranged from 1.1 to 4.5 mm. The water balance tended to overestimate ET during winter, by assuming deep drainage was zero, whereas the energy balance tended to overestimate ET very late in the season. For duplex soils in south-western Australia, a composite approach may give a cheap and accurate estimate of growing season ET, provided that rainfall is adequate to maintain surface soil moisture. This approach assumes that ET equals potential ET until the rapid decline in soil water begins, and ET equals changes in soil water storage after this time.     

Estimating spatial mean soil water contents of sloping jujube orchards using temporal stability

Estimating spatial mean soil water contents from point-scale measurements is important to improve soil water management in sloping land of semiarid areas. Temporal stability analysis, as a statistical technique to estimate soil water content, is an effective tool in terms of facilitating the upscaling estimation of mean values. The objective of this study was to examine temporal stability of soil water profiles (0–20, 20–40, 40–60 and 0–60 cm) in sloping jujube (Zizyphus jujuba) orchards and to estimate field mean root-zone soil water based on temporal stability analysis in the Yuanzegou catchment of the Chinese Loess Plateau, using soil water observations under both dry and wet soil conditions. The results showed that different time-stable locations were identified for different depths and the temporal stability of soil water content in 20–40 cm was significantly (P < 0.05) weaker than that in other depths. Moreover, these time-stable locations had relatively high clay contents, relatively mild slopes and relatively planar surfaces compared to the corresponding field means. Statistical analysis revealed that the temporal stability of root zone soil water (0–60 cm) was higher in either dry or wet season than that including both, and soil water exhibited very low temporal stability during the transition period from dry to wet. Based on the temporal stability analysis, field mean soil water contents were estimated reasonably (R² from 0.9560 to 0.9873) from the point measurements of these time-stable locations. Since the terrains in this study are typical in the hilly regions of the Loess Plateau, the results presented here should improve soil water management in sloping orchards in the Loess Plateau.     

Daily evapotranspiration estimates from extrapolating instantaneous airborne remote sensing ET values

In this study, six extrapolation methods have been compared for their ability to estimate daily crop evapotranspiration (ET_d) from instantaneous latent heat flux estimates derived from digital airborne multispectral remote sensing imagery. Data used in this study were collected during an experiment on corn and soybean fields, covering an area of approximately 12 × 22 km, near Ames, Iowa. ET_d estimation errors for all six methods and both crops varied from −5.7 ± 4.8% (MBE ± RMSE) to 26.0 ± 15.8%. Extrapolated ET_d values based on the evaporative fraction (EF) method better compared to eddy covariance measured ET values. This method reported an average corn ET_d estimate error of −0.3 mm day⁻¹, with a corresponding error standard deviation of 0.2 mm day⁻¹, i.e., about 5.7 ± 4.8% average under prediction when compared to average ET_d values derived from eddy covariance energy balance systems. A solar radiation-based ET extrapolation method performed relatively well with ET_d estimation error of 2.2 ± 10.1% for both crops. An alfalfa reference ET-based extrapolation fraction method (ET_rF) yielded an overall ET_d overestimation of about 4.0 ± 10.0% for both crops. It is recommended that the average daily soil heat flux not be neglected in the calculation of ET_d when utilizing method EF. These results validate the use of the airborne multispectral RS-based ET methodology for the estimation of instantaneous ET and its extrapolation to ET_d. In addition, all methods need to be further tested under a variety of vegetation surface homogeneity, crop growth stage, environmental and climatological conditions.

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Short term effects of saline irrigation on evapotranspiration from lysimeter-grown citrus trees

Eight-year-old Murcott orange trees grown in greenhouse lysimeters filled with sandy soil were subjected to irrigation with saline water to investigate the influence of salinity on daily evapotranspiration (ET). The study was conducted in Japan from 1 August to 15 September 2000. The study duration was divided into three periods of about 2 weeks each. In period I, all lysimeters planted with a tree were irrigated with 60 mm of non-saline water at the water content of 70% of field capacity (FC). Salinity treatments for period II started on 14 August. The treatments during period II were as follows: Lysimeter 1 (L1) had 32 mm non-saline water with an electrical conductivity (EC_I) of 1.0 dS/m applied. At the same time Lysimeter 2 (L2) had 32 mm of saline water with an EC_I of 8.6 dS/m applied when the water content decreased to 70% of FC. Lysimeter 3 (L3) had 16 mm saline water (EC_I=8.6 dS/m) applied at 85% of FC. The irrigation amounts during period II were equal to those corresponding to 1.2 times of water required to reach FC. Treatments in period III were the same as in period I.Daily ET was similar for all weighing lysimeters during period I. The average relative ET for L2 and L3 with respect to L1 (L2/L1 and L3/L1) were similar during this period, with a mean value of 0.99. During period II, ET from L1 was consistently higher than that from L2 and L3. In addition, L3 with a higher irrigation frequency because of irrigation at higher soil water content resulted in higher ET than L2. The average relative ET of period II was 0.71 and 0.88 for both L2 and L3. During the last half of period III, reductions occurred in the ET differences between the saline treatments (L2 and L3) and non-saline control (L1).Evaporation rates from soil did not exceed 0.7 mm per day. Transpiration rates from L1, L2 and L3 during period II varied between 6.3 and 3.1 mm per day, 4.5 and 2.2 mm per day, and 5.8 and 3.0 mm per day, respectively. The results reflected a tangible difference of water extraction by roots from individual soil layers. Maximum water uptake by these trees was observed at layer of 30–60 cm. Nevertheless, no clear differences in water extraction pattern between trees were observed.Approximately, 95% of drainage occurred during the first 2 days following irrigation. The electrical conductivity of soil water (EC_S) and the electrical conductivity of drainage water (EC_D) for the saline water treatments (L2 and L3), compared to the control (L1) were significantly different during period II. EC_S values were 2–5 times higher in saline treatments compared to the control treatment. After irrigating trees with saline water, EC_S increased from 5 to 14 and 16 dS/m in L2 and L3, respectively. Similarly, in both saline treatments, EC_D values were greatly increased after irrigation. During period III, EC_D values increased from 5 to 8 dS/m in L2, and from 3 to 11 dS/m in L3. By contrast, EC_S declined from 14 to 5 dS/m in L2, and from 16 to 3 dS/m in L3 over the same period.     

Evaluation of satellite evapotranspiration estimates using ground-meteorological data available for the Flumen District into the Ebro Valley of N.E. Spain

Accurate estimation of actual evapotranspiration (ET_a) is essential for effective local or regional water management. At a local scale, ET estimates can be made accurately considering a soil-plant-atmospheric system, if adequate meteorological-ground data or ET measurements are available. However, at a regional scale, ET_a values cannot be measured directly and the estimates are frequently subject to errors. Although it is possible to extrapolate to the regional scale from local (point) data of meteorological stations, the relative sparse coverage of ground estimate can make this problematic without some spatial analysis to demonstrate the similarity of the climate in the area. The introduction of remote sensing data and techniques offers alternatives both to estimate variables (i.e. radiation) and parameters (i.e. ET) with few spatial restrictions, thus, it provides potential advantages to the regional ET_a computation. In particular, the use of remote sensing procedures like the surface energy balance-based algorithms (SEB) have been successfully applied in different climates, enabling the estimation of ET_a at local and regional scales. A proper variation of the Surface Energy Balance Algorithm for Land (SEBAL) was applied to 4 years of data for the Flumen District in the Ebro Basin at the N.E. of Spain. Results obtained show that the remote sensing algorithm can provide accurate daily ET_a estimations as compared with lysimeter measurements of daily ET values for two crop plots: one with a reference grass and other with maize or wheat as function of the season. Also a comparison between ET_a and the reference and crop ET values applying the Penman-Monteith method was carried out. The comparison analysis consider an accepted error difference of 1.0 mm d⁻¹ (20% of error) for local scale, the ET_a values for the grass show a bias of 0.30 mm d⁻¹ against the ET_grass and a bias of 0.36 mm d⁻¹ against ET_o. Differences between ET_maize or ET_wheat and ET_a against their average showed an acceptable agreement for the field with s_diff ± 0.6 mm d⁻¹. For the crop fields at regional scale external causes associated to atmospheric and surface variations (i.e. land preparation) rather to the remote sensing algorithm made difficult to determine a percentage of error. Finally, ET_a values were obtained at regional scale and it was demonstrated that using the remote sensing improve significantly the crop ET estimations computed in the area using traditional methods.     

基于HJ-1卫星影像的三大农作物估产最佳时相选择

对于农作物遥感估产,精确选择最佳估产时相是关键环节。该文利用中国自行研发的HJ-1卫星CCD影像对黑龙江八五二农场3大作物（水稻、玉米、大豆）进行遥感估产的最佳时相选择,通过构建小波变换滤波方法和移动平均法的时序NDVI曲线数据,并依据平滑后的时序NDVI曲线分别确定3大作物的遥感估产最佳时相。研究结果表明,从平滑后的时序NDVI曲线中识别出来的3大作物的关键生长期与当地作物的物候期相对比,水稻生长期拟合误差为-0.003356508,玉米生长期拟合误差为-0.001687117,大豆生长期拟合误差为-0     

Relations between available and extractable soil water and evapotranspiration from a bean crop

The accuracy of ‘available’ and ‘extractable’ soil water estimates was investigated using irrigated and unirrigated beans (Vicia faba) grown in an alluvial silt loam in Canterbury, New Zealand. Available water capacity was defined as the difference between soil water contents in the root zone at the drained upper limit (DUL) and at the lower limit (LL) as estimated by laboratory procedures. Extractable water capacity was specified as the difference between field estimates of DUL and LL for the whole profile affected by roots. DUL was estimated in the laboratory by equilibrating soil cores at matric potentials at ?10, ?20 or ?30 kPa, and in the field by neutron moderation. Laboratory estimates of LL were made from soil samples equilibrated at ?1.5 MPa matric potential. In the field LL was measured by neutron moderation on plots where evaporation had apparently ceased due to drought stress.When compared at intervals down the profile laboratory estimates of DUL and LL showed poor agreement with field observations. However, the final estimates of available and extractable water capacities were similar because of compensatory inaccuracies in the laboratory estimates. Furthermore, field measurements of evapotranspiration, using neutron moderation and tensiometry, indicated that the accuracy of the available water estimates was much reduced by upward fluxes of water into the rooting zone. These fluxes resulted in water extraction to at least 1.0 m although the apparent maximum rooting depth (measured by counting roots washed from soil cores) was only 0.7 m.Particular attention was paid to the influence of subsoil textural variability, which is pronounced in such soils. Laboratory and field estimates of the LL had to be carefully matched texturally before relevant comparisons could be made. Problems associated with subsoil textural variability affected laboratory methods of DUL estimation more than field methods.     

Determination of evapotranspiration from an alfalfa crop irrigated with saline waste water from an electrical power plant

Summary Investigations were carried out in 1989 to determine the evapotranspiration (ET) of alfalfa when irrigated with saline waste water coming from the evaporation of fresh water in the cooling towers of Utah Power and Light Company Electrical Power Plant at Huntington in central Utah, U.S.A. The primary goal is to dispose of the waste water from the power plant by irrigation and to maximize salt deposition in the soil, maximize crop ET, minimize runoff from the soil surface, and minimize leaching to the ground water. Using the Bowen ratio-energy balance method, alfalfa evapotranspiration was measured at an experimental site for each 20-minute period during the 1989 irrigation season. Using a simplified seasonal water balance, the results showed that cumulative irrigation plus rain was less than evapotranspiration for the 1989 irrigation season. This means that for the long term in addition to irrigation and precipitation some water was withdrawn from the soil for alfalfa crop water requirements (ET_a). Short term evaluations showed that because of unforeseen heavy rain (thunder showers) in this mountainous area between irrigations, ET_a was occasionally less than irrigation plus rain. This means the excess water was stored in the soil for later use. The average value for ET_a/ET_p (potential ET) for the 1989 irrigation season was 0.47 but occasionally the ratio was greater than unity. Short-term studies (Hanks et al. 1990 a) indicate that yield and ET_a are likely to decrease only slightly for the coming years if saline irrigation water is applied. This method of investigation can be applied to any industrial processes which produce waste water.     

Water table behaviour in drained lands: effect of evapotranspiration from the water table

Subsurface drain spacing is underestimated by the equations that do not account for evaporation-evapotranspiration (ET) lowering the water table in drained lands. An analytical solution is proposed to evaluate water table behaviour in subsurface drained lands in the presence of ET. A piecewise linear model is proposed and used to describe any realistic functional relation between ET and depth to water table. Characteristics of the solution have been highlighted with the help of numerical examples for which drainage parameters have been chosen from two actually operating drainage systems installed in semi-arid regions. The accuracy of the proposed solution has been verified with the existing numerical scheme as well as by comparing the water table behaviour with the observed field data. Application of the solution in subsurface drainage design has been illustrated which suggests that drain spacing at this particular site could be increased by 9 to 18% if the contribution of ET in lowering the water table is taken into account.     

Numerical assessment of effective evapotranspiration from maize plots to estimate groundwater recharge in lowlands

To maximize the irrigation efficiency and to protect groundwater from agrochemical pollution, two variables must be known with good accuracy: effective evapotranspiration and infiltration, especially in lowland areas were the run-off is minimal. Three different experimental plots cultivated with maize were equipped with tensiometers and soil moisture probes to monitor every day the water movement in the unsaturated zone. Other relevant parameters of the various soil layers, as hydraulic conductivity and water retention curve, were obtained in laboratory experiments, while boundary conditions, as precipitations, temperature and root growth, were obtained on site. Inverse modeling was performed using HYDRUS-1D to assess the degree of uncertainty on model parameters. Results showed a good model fit of water content and head pressure at various depths, in each site, using Penman-Monteith formula for daily potential evapotranspiration calculation, but poor fit applying the Hargreves and Turk formulas. Best performance of model fit was observed for S-shaped equation employed to simulate the root water-uptake reduction with respect to Feddes equation. The soil parameters uncertainty was limited and remained within analytical errors, thus a robust estimation of cumulative infiltration and evapotranspiration has been derived. This study points out that evapotranspiration is the most important variable in defining groundwater recharge for maize crops in lowlands.     

Artificial neural network estimation of reference evapotranspiration from pan evaporation in a semi-arid environment

The objective of this study was to test an artificial neural network (ANN) for converting pan evaporation data (E _p) to estimate reference evapotranspiration (ET₀) as a function of the maximum and minimum air temperature. The conventional method that uses Pan coefficient (K _p) as a factor to convert E _p to ET₀, is also considered for the comparison. The ANN has been evaluated under semi-arid conditions in Safiabad Agricultural Research Center (SARC) in the southwest of Iran, comparing daily estimates against those from the FAO-56 Penman–Monteith equation (PM), which was used as standard. The comparison shows that, the conventional method underestimated ET₀ obtained by the PM method. The ANN method gave better estimates than the conventional method that requires wind speed and humidity data.     

Estimating the potential gains from water markets: a case study from Tunisia

Water markets can improve water use efficiency through the transfer of water to users who can obtain the highest marginal return from using it. Existing water markets are implemented among farmers or between farmers and urban water companies or hydropower companies. Several studies have shown that farmers may benefit from trading water mainly in countries where water scarcity is increasing and new water supply projects are either very costly or not possible because of environmental concerns. This paper estimates the potential benefits and losses of implementing water market among farmers and between farmers and urban water company in Tunisia. We used linear programming to examine four separate farm models and an aggregate model. The method is applied to an irrigation area of 4500 ha in Northern Tunisia. Results indicate that water trading among farmers would be quite limited and would have a minor impact on farmers’ income. In contrast, the market among farmers and the urban water company offers higher volumes of water trades to urban users and helps increase farmers’ profitability by up to 7.9%. The sale of water to the urban company is accompanied by a decrease in occasional labor by as much as 34.8% and a decrease of up to 17.6% in farmers’ expenditures for inputs and machinery. Additionally, results obtained in this paper show that inter-year storage of irrigation water may be more advantageous than selling water to the urban utility. Whether farmers would opt to sell water or inter-temporarily store it would depend on the establishment of water rights and the empowerment of farmers.