Maximum Evapotranspiration Rates Research Articles

Yield response factor (Ky) and initial growth in black pepper in a tropical environment

Black pepper is widely produced in Brazil, though there are few studies related to its water requirements and the effects of water deficit (WD) applied under tropical conditions. The growth, sensitivity to WD, and first-year productivity of irrigated plants from the Bragantina cultivar were evaluated, employing an irrigation system with automatic management. The plants were cultivated in plastic pots installed in the field for 391 days using emitters with different flow rates (2.2 [T1], 3.3 [T2], 4.3 [T3], and 5.3 L h–1 [T4 – control]). Fresh weight of the leaves, stems, and shoots, and dry weight of the leaves, stem, roots, and shoots as well as the total dry weight of the plant, were evaluated. Productivity was determined by evaluating the grains and spikes from a single harvest. Yield results and the actual versus maximum evapotranspiration rate for the entire growing season was used for calculating the Ky coefficient. Despite 1,346 mm of rainfall, the irrigation system was activated 165 times, distributing a mean volume of 19.0 (T1), 28.5 (T2), 37.1 (T3), and 45.8 L per plant (control). All traits were negatively affected by the WD, and the Ky values obtained (from 1.72 to 2.96) indicate the high sensitivity of black pepper to WD. In general, the spikes produced with at least 81 % of the crop water demand were larger and more numerous than those subjected to inferior treatments. WD occurring during the flowering stage severely hampers the size, weight, and quality of the spikes and grains of black pepper.

Land suitability assessment for grapevines via laser level in water-scarce regions of Uzbekistan (in the case of Kashkadarya province)

The most suitable arable land for viticulture is mainly located in the rain-fed areas such as in the foothills and mountainous regions of Uzbekistan where vineyards reach the acme point of the yield productivity. Irrigating with rainwater in the vineyard has a great potential for obtaining stable yields at the low cost of irrigation water which is proven by experimental and theoretical studies. This paper aims at implementing and testing the advantage of sustainable irrigation methods in water-scarce regions of Uzbekistan by scrutinizing the hydraulic characteristics of irrigation water and slope of the study area. We considered the maximum evapotranspiration rate in general for all phases to calculate the amount of irrigation water required per one vine tree since considering the water scarcity in the maximum growing season is important. We firstly aimed at constructing the pool to store raindrops. To perform this, we used the laser level for checking land suitability and a standardized 20 Ø irrigation pipe to deliver rainwater from the pool to the furrows of the vineyard. The results of the geodetic survey showed that the average slope of the pilot area was equal to 0.022 and enabled to implement the rainwater irrigation.

Evapotranspiration estimation using a modified Priestley-Taylor model in a rice-wheat rotation system

The rice-wheat rotation system is one of the largest agricultural production systems worldwide. Accurate estimation of evapotranspiration (ET) in the rice-wheat rotation system is critical to enhance efficient irrigation management and water use. The variation of ET for a rice-wheat rotation system during 2015–2018 and its controlling meteorological factors was investigated using the Bowen ratio energy balance and path analysis methods. A modified Priestley–Taylor (PT) model considering soil water stress for soil evaporation (E) (fsw), and plant temperature constraint (deviation of air temperature from optimum for the crops used, ft), leaf senescence for transpiration was developed. The results showed that the diurnal variation of ET rate in different months exhibited a single peak curve with the maximum ET rates of 0.90 and 0.42 mm h−1 for rice and winter wheat, respectively. The total ET of the rice-wheat rotation system over the whole growing season was 765–841 mm, of which 63–67% was consumed by the rice field. The average daily ET rate over the whole growing season was 3.27–4.13 and 1.50-1.65 mm d−1 for rice and winter wheat, respectively. The results of ET partitioning showed that E accounted for 23–32% of the seasonal ET for rice and 48–51% for winter wheat. The ET partitioning of rice and winter wheat was closely linked to leaf area index (LAI). The ratio of E/ET reduced exponentially for rice with the increase of LAI, while it reduced linearly for winter wheat. The path analysis showed that the net radiation (Rn) was the dominant meteorological factor affecting short-term ET of the rice-wheat rotation system through the direct effect. The water vapour pressure deficit (VPD), another important factor influencing ET, showed mainly an indirect effect on ET through path of Rn and had a greater impact on ET for rice than that for wheat. The modified PT model could estimate ET for rice and winter wheat reasonably, with linear regression coefficient of 0.93–1.09 and coefficient of determination of 0.92−0.96. The model was sensitive to the fsw or ft.

Intercomparison of Model-Estimated Potential Evapotranspiration on the Tibetan Plateau during 1981–2010

Abstract Potential evapotranspiration (PET), the maximum evapotranspiration rate under unlimited water supply, reflects the capacity for transpiration flow and plant primary production. Numerous models have been developed to quantify PET, but there are still large uncertainties in PET estimations. In this study, the authors conducted spatially explicit estimations of daily PET from 1981 to 2010 for eight different land-cover types on the Tibetan Plateau by applying three types of PET models including a combination model (Penman–Monteith), a radiation-based model (Priestley–Taylor), and a temperature-based model (Thornthwaite). This study found that the PET estimated by Thornthwaite model (PETT) was lower than those estimated by Priestley–Taylor (PETPT) and Penman–Monteith models (PETPM). Penman–Monteith model gave the highest estimates of PET on annual and daily scales. The mean annual PET for the whole plateau estimated by these three models varied from 675.1 to 700.5 mm yr−1, and daily PET varied from 1.33 to 1.92 mm day−1. The spatial pattern of PETT did not agree with the PETPT and PETPM, while the latter two agreed well with each other. Because of different model structures and dominant meteorological drivers, the interannual variability of PET varied significantly among the models. PETPT and PETPM showed a transition around 1993 since the dominant meteorological drivers were different before and after 1993. These disagreements among different models suggested that PET models with different algorithms should be used with caution. This study provided a validation to assist those undertaking PET estimations on the Tibetan Plateau.

Soil Moisture Stochastic Model in Pinus tabuliformis Forestland on the Loess Plateau, China

As an important restrictive factor of ecological construction on the Loess Plateau, the study of soil moisture dynamics is essential, especially under the impact of climate change on hydrological processes. In this study, the applicability of the Laio soil moisture stochastic model on a typical plantation Pinus tabuliformis forestland on the Loess Plateau was studied. On the basis of data concerning soil properties, climate, and plants of the typical forestland during the period 2005–2015 in the Chinese National Ecosystem Research Network (Ji County Station) in Ji County, Shanxi, model results were acquired and compared with observed soil moisture from 2005 to 2015 in the study area. The genetic algorithm method was used to optimize model parameters in the calibration process. In the calibration and validation periods, the relative error between numerical characteristics of simulated and observed soil moisture values was mostly within 10%, and model evaluation index J was close to 1, indicating that the Laio model had good applicability in the study area. When calibrating the model, it was recommended to use soil moisture data with a sampling interval of no more than 10 days so as to reduce the loss of soil moisture fluctuation information. In the study area, the Laio model was strongly sensitive to variations of input parameters, including maximum evapotranspiration rate Emax, average rainfall depth α, and average rainfall frequency λ, which should be paid more attention for stable and reliable simulation results. This study offers a method to obtain soil moisture data at ungauged sites. Results from this study provide guidance for Laio model application on the Loess Plateau.

Extreme precipitation drives groundwater recharge: the Northern High Plains Aquifer, central United States, 1950–2010

AbstractFuture extreme precipitation (EP, daily rainfall amount over certain thresholds) is projected to increase with global climate change; however, its effect on groundwater recharge has not been fully explored. This study specifically investigates the spatiotemporal dynamics of groundwater recharge and the effects of extreme precipitation (daily rainfall amount over the 95th percentile, which is tagged by ranking the percentiles in each season for a base period) on groundwater recharge from 1950 to 2010 over the Northern High Plains (NHP) Aquifer using the Soil Water Balance Model. The results show that groundwater recharge significantly (p < 0.05) increased in the eastern NHP from 1950 to 2010, where the highest annual average groundwater recharge occurs compared to the central and the western NHP. In the eastern NHP, 45.1% of the annual precipitation fell as EP, which contributed 56.8% of the annual total groundwater recharge. In the western NHP, 30.9% of the annual precipitation fell as extreme precipitation, which contributed 62.5% of the annual total groundwater recharge. In addition, recharge by extreme precipitation mainly occurred in late spring and early summer, before the maximum evapotranspiration rate, which usually occurs in mid‐summer until late fall. A dry site in the western NHP and a wet site in the eastern NHP were analysed to indicate how recharge responds to EP with different precipitation regimes. The maximum daily recharge at the dry site exceeded the wet site when there was EP. When precipitation fell as non‐extreme rainfall, most recharge was less than 5 mm at both the dry and wet sites, and the maximum recharge at the dry site became lower than the wet site. This study shows that extreme precipitation plays a significant role in determining groundwater recharge. © 2016 The Authors Hydrological Processes Published by John Wiley & Sons Ltd.

Determination of the potential evapotranspiration and crop coefficient for saffron using a water-balance lysimeter

Agriculture is the major consumer of water and it is possible to decrease water consumption in this sector by proper irrigation scheduling. Irrigation scheduling is based on crop water requirements. Saffron is an important crop in Iran. The main purpose of this study was to determine the potential evapotranspiration and crop coefficient for saffron using single and dual crop coefficients, in Badjgah region, College of Agriculture, Shiraz University, Shiraz, Iran. Three water-balance lysimeters were used for this experiment in a two-year study. Total saffron potential evapotranspiration values were 523 and 640 mm in the first and second growing seasons, respectively. The maximum evapotranspiration rates for saffron were 4.5 and 6.1 mm d−1 in the first and second growing seasons, respectively. Based on the results of this study, different saffron growing stages for evapotranspiration were 30, 40, 70 and 60 days. Crop coefficient (K c) values for the initial, mid- and late-season growth stages were 0.41–0.45, 0.93–1.05 and 0.29–0.31 in both years, respectively. Basal crop coefficient (K cb) values for the initial, mid- and late-season growth stages were 0.15–0.16, 0.41–0.65 and 0.15–0.17 in both years, respectively.

Comparison of ecosystem water‐use efficiency among Douglas‐fir forest, aspen forest and grassland using eddy covariance and carbon isotope techniques

AbstractComparisons were made among Douglas‐fir forest, aspen (broad leaf deciduous) forest and wheatgrass (C3) grassland for ecosystem‐level water‐use efficiency (WUE). WUE was defined as the ratio of photosynthetic CO2 assimilation rate and evapotranspiration (ET) rate. The ET data measured by eddy covariance were screened so that they overwhelmingly represented transpiration. The three sites used in this comparison spanned a range of vegetation (plant functional) types and environmental conditions within western Canada. When compared in the relative order Douglas‐fir (located on Vancouver Island, BC), aspen (northern Saskatchewan), grassland (southern Alberta), the sites demonstrated a progressive decline in precipitation and a general increase in maximum air temperature and atmospheric saturation deficit (Dmax) during the mid‐summer. The average (±SD) WUE at the grassland site was 2.6±0.7 mmol mol−1, which was much lower than the average values observed for the two other sites (aspen: 5.4±2.3, Douglas‐fir: 8.1±2.4). The differences in WUE among sites were primarily because of variation in ET. The highest maximum ET rates were approximately 5, 3.2 and 2.7 mm day−1 for the grassland, aspen and Douglas‐fir sites, respectively. There was a strong negative correlation between WUE and Dmax for all sites. We also made seasonal measurements of the carbon isotope ratio of ecosystem respired CO2 (δR) in order to test for the expected correlation between shifts in environmental conditions and changes to the ecosystem‐integrated ratio of leaf intercellular to ambient CO2 concentration (ci/ca). There was a consistent increase in δR values in the grassland, aspen forest and Douglas‐fir forest associated with a seasonal reduction in soil moisture. Comparisons were made between WUE measured using eddy covariance with that calculated based on D and δR measurements. There was excellent agreement between WUE values calculated using the two techniques. Our δR measurements indicated that ci/ca values were quite similar among the Douglas‐fir, aspen and grassland sites, despite large variation in environmental conditions among sites. This implied that the shorter‐lived grass species had relatively high ci/ca values for the D of their habitat. By contrast, the longer‐lived Douglas‐fir trees were more conservative in water‐use with lower ci/ca values relative to their habitat D. This illustrates the interaction between biological and environmental characteristics influencing ecosystem‐level WUE. The strong correlation we observed between the two independent measurements of WUE, indicates that the stable isotope composition of respired CO2 is a useful ecosystem‐scale tool to help study constraints to photosynthesis and acclimation of ecosystems to environmental stress.

Potato and Human Exploration of Space: Some Observations from NASA-Sponsored Controlled Environment Studies

Future space exploration by humans will require reliable supplies of food, oxygen and clean water to sustain the expeditions. Potato is one of several crops being studied for such a “life support” role. Tests sponsored by the US National Aeronautics and Space Administration (NASA) confirmed the well-known short day tendencies for tuberisation, but also revealed that some cvs. (e.g., Norland, Denali and Russet Burbank) could tuberise well under continuous high light. Horticultural tests showed that plants grew well and tuberised readily using a nutrient film technique (NFT). CO2 enrichment studies with potato showed typical C3 responses in photosynthesis and yield, with maximum rates occurring near 1000 μmol mol−1. The highest tuber yields from these controlled environment studies reached 19.7 kg FM m−2 or equivalent to nearly 200 t ha−1. This equated to a productivity of 38 g m−2 DM m−2 day−1. Stand evapotranspiration (ET) rates ranged from 3.4 to 5.2 l m−2 day−1 throughout growth, while maximum ET rates for canopies could approach 10 l m−2 day−1. Harvest indices (tuber DM/total DM) typically ranged from 0.7 to 0.8, indicating that waste (inedible) biomass from potato would be less than that from many other crops. An experiment was conducted in 1995 on NASA’s Space Shuttle using excised potato leaves to study tuber formation at axillary buds during spaceflight. The results showed that tubers formed equally well in space as in the ground controls, indicating that reduced gravity should not be an impediment to tuberisation.

Carbon Dioxide and Temperature Effects on Evapotranspiration and Water Use Efficiency of Soybean

Rising CO2 and potential global warming will cause changes in evapotranspiration (ET). Our research objective was to determine the impact of CO2 and air temperature on canopy ET, water use efficiency (WUE), foliage temperature, and canopy resistance (Rc) of soybean [Glycine max (L.) Merr.]. Plants were grown in sunlit, controlled‐environment chambers at cyclic maximum/minimum air temperatures from 28/18°C to 44/34°C and CO2 of 350 or 700 μmol mol−1 Maximum ET rate in the early afternoon at 35 d after planting ranged from 7.5 mol m−2 s−1 at 28/18°C to 19.0 mol m−2 s−1 at 44/34°C. Daily ET during the middle of the season ranged from 260 mol H2O m−2 d−1 (4.7 mm d−1) at 28/18°C to 660 mol H2O m−2 d−1 (11.9 mm d−1) at 44/34°C. Mean daily ET was linearly related to mean air temperature (Tair) as: [Mean daily ET = 21.4 × Tair − 306, r2 = 0.99 (mol H2O m−2 d−1), or mean daily ET = 0.385 × Tair − 5.5 (mm d−1)]. Doubled CO2 caused a 9% decrease in ET at 28/18°C, but CO2 had little effect at 40/30°C or 44/34°C. Whole‐day WUE declined linearly with air temperature, with a slope of −0.150 [(μmol CO2 mmol−1 H2O) °C−1]. Changes in ET and WUE were governed by changes in foliage temperature and Rc. In conclusion, increases in temperature anticipated by climate change could more than offset decreases of ET that would be caused by increases in CO2

Using the CERES-Maize model in a semi-arid Mediterranean environment. New modelling of leaf area and water stress functions

The CERES-Maize model does not simulate satisfactorily the productivity under soil water scarcity. In a previous paper by Ben Nouna et al. (Eur. J. Agron. 13 (2000) 309), this poor performance was attributed to an inaccurate leaf surface simulation and to the water stress function. This paper proposes the introduction of two new modules in the original version of CERES-Maize model. For the LAI module, an analytical approach is followed, consisting of simulating the leaf area of each individual leaf, by the calibration of different functions (leaf appearance, leaf area growth, end of leaf growth, leaf senescence). The new stress function expresses the ratio of actual to maximum evapotranspiration rate as a function of an indicator of the plant water status (pre-dawn leaf water potential). The two modules have been separately introduced into the CERES model and the output compared with the observed values and those simulated by the CERES model in its original version. The results of validation showed that the revised LAI module improved the LAI prediction compared with the one of the original model. Moreover, for the purpose of generalising the new LAI module, its predictions were also compared with the LAI simulated by the Muchow et al. model (Agron. J. 87 (1990) 338), which has been calibrated at several latitudes with different maize varieties. As compared with the original version of CERES, the new water stress function gave a different prediction in terms of the timing and intensity of soil water deficit during the cropping cycle. The stress intensity, as calculated by the new function, was generally less severe than that simulated by the original version.

Water flow modeling in the unsaturated zone with imprecise parameters using a fuzzy approach

Abstract An alternative approach, based on fuzzy set theory, is presented to express imprecision of parameters in a non-probabilistic sense. Imprecision may originate from indirect measurements, estimation routines, subjective interpretation and expert judgement of available information. One dimensional, steady state water flow in the unsaturated zone of homogeneous soils, which is described by the Darcy-Buckingham equation, was chosen to evaluate and to incorporate fuzzy soil hydraulic properties and boundary conditions in the modeling procedure. It is here used to describe soil water pressures with depth, as well as to calculate maximum evapotranspiration rates under steady state conditions. Solving the fuzzy equation for steady state water flow results in minimizing/maximizing procedures, from where resulting membership functions of the dependent variable are calculated. A comparison to a more classical stochastic approach points out the main differences between fuzzy and stochastic concepts to account for uncertainties. Finally, a sensitivity analysis shows the strong impact of different shapes of membership functions of the input parameters on the resulting membership functions of maximum evapotranspiration rates and soil water pressures.

Evaluation of sugarcane evapotranspiration from water table data in the everglades agricultural area

The limited fresh water supply in south Florida has emerged as a major point of contention between the urban, environmental, and agricultural sectors. The mandated water quality improvement measures suggest the retention of water on farms (up to 20% of historical drainage pumping) could further impact the regional water supply. Accurate measures of evapotranspiration (ET) are vital in order to prudently plan for the equitable distribution of water. The daily changes in field water table depths were used to evaluate the actual evapotranspiration for sugarcane grown in the Histosols of the Everglades Agricultural Area (EAA) in south Florida. Water table recorders were installed in eight independently irrigated and drained plots, each 0.55 ha. Water table monitoring was conducted for a period of two years, 1990 and 1991, and the results were the average of these two years. The crop was plant and first ratoon. Each month, periods of time when no rainfall, irrigation, or drainage occurred were selected for use in ET determination. The time periods ranged from 5 to 19 days. Assuming that there were no seepage or deep percolation the daily declines in water tables, which exhibited clear diurnal patterns, were then used to determine the change in soil water losses during the selected time periods, the change in soil water storage was representative of actual sugarcane ET. Pan evaporation and crop growth were also monitored. The amount of water released from the soil during drainage was determined in laboratory columns. Five transparent PVC pipes, 12.1 cm in diameter were used to remove undisturbed 110 cm deep cores from the field. The columns of soil were saturated and drained to determine the drainable porosity of the soil. The water table data exhibited diurnal trends, with declines close to zero at midnight and maximum declines occurring between 2:00 and 3:00 pm. The data showed that minimum ET rates occurred during December through February (0.7–1.5 mm day t−1 ). Maximum ET rates (4.5–4.6 mm day t−1 ) occurred during June through September. Total ET was 106.2 cm per year. Using the actual field measured ET, crop coefficients (K c) were calculated for sugarcane for the Penman, Thornthwaite, and Blaney-Criddle methods of determining potential evapotranspiration (ET p).

Net Photosynthetic and Evapotranspiration Rates in Situ of Tomato and Lettuce Plug Sheets in the Greenhouse

Net photosynthetic and evapotranspiration rates of tomato (LAI = 2.3) and lettuce (LAI = 6.6) plug sheets were estimated based on measurements of the weight of plug sheets and vertical profiles of CO2 concentration above the plug sheets. The measurements were continued in situ for several days in a greenhouse when plugs were at transplant stage. The maximum net photosynthetic rates of tomato and lettuce plug sheets were 0.8 and 2.0 mg CO2/m2 per sec on a plug sheet area basis, respectively. The maximum evapotranspiration rates of those sheets were 100 mg·m–2·s–1. Net photosynthetic and evapotranspiration rates of tomato and lettuce plug sheets increased linearly with an increase in solar radiation flux, with a correlation coefficient of 0.9.

Comparing Turfgrass Cumulative Evapotranspiration Curves

Twenty-five commercially available turfgrass cultivars were evaluated for cumulative evapotranspiration (ETcum) attributes under progressive water stress for 0 to 21 and 0 to 24 days using the gravimetric mass balance method in two greenhouse studies. At the end of the water-stress treatment, the cultivars were scored visually for their green appearance on a 0 (no green) to 10 (100% green) scale. The Gompertz nonlinear model gave a best fit to ETcum vs. days adjusted for pan evaporation variation in the greenhouse compared with monomolecular and logistic nonlinear regression models. Two ETcum attributes—maximum evapotranspiration rates (ETmax) and inflection time (ti) (the time when the change in ET becomes zero)—were estimated for each cultivar using the Gompertz model. Based on final ETcum, ETmax, ti, and greenness score, `Bristol', `Challenger', and `Wabash' Kentucky bluegrass (Poa pratensis L.); `Shademaster' creeping fescue (Festuca rubra L.); `FRT-30149' fine fescue (F. rubra L.); and `Aurora' hard fescue (F. ovina var. duriuscula L. Koch.) were identified as low water-use cultivars.

Rooting Activity and Water Use during Vegatative Development of Crested and Western Wheatgrass

AbstractCrested wheatgrass [Agropyron desertorum (Fisch. ex. Link) Schult] and western wheatgrass [Pascopyrum smithii Rybd, (Löve)] are important cool‐season forage species that have contrasting growth patterns. Knowledge of relationships among plant development stage, water use, and rooting activity would aid in understanding the responses of these species to stresses and defoliation. Our objectives were to compare evapotranspiration (ET), rooting activity, and plant water status in relation to Haun development stage of ‘Nordan’ crested wheatgrass and ‘Rodan’ western wheatgrass. These species were grown at three water treatments (50, 100, and 150% of 1 April to 31 October monthly rainfall) and two N fertilizer (11 and 110 kg N ha−1) rates in a rain‐out shelter at Mandan, ND for a 3‐yr period. The soil was a Parshall fine sandy loam (coarse‐loamy, mixed Pachic Haploborolls). Cumulative ET through early boot stage averaged 133 and 170 mm for crested and western wheatgrass, respectively. Maximum ET rate occurred at Haun stage 5 to 6 for both species. Crested wheatgrass rooting activity averaged over three water treatments increased about 4.4 cm d−1 compared to 3.7 cm d−1 for western wheatgrass. Both species exhibited rooting activity to 1.2 m by mid‐May when western wheatgrass had developed to Haun stage 4.9 and crested wheatgrass to 5.7. Water use‐efficiency of crested wheatgrass decreased as water treatment and Haun stage increased, but western wheatgrass did not change with either. Water use‐efficiency of crested wheatgrass, but not of western wheatgrass, was greater at 110 than at 11 kg fertilizer N ha−1. Information on the simultaneous development of plant aerial parts and rooting activity contributes to an understanding of the water‐use relations of these species and has application in development of growth and yield simulation models.

Estimating evapotranspiration from wheat using weather measurements and carborundum or Piché evaporimeter

Hourly evaporation rates from a Piché evaporimeter installed inside a Stevenson screen were compared with rates measured from a Piché exposed to the atmosphere, but screened from direct short- and long-wave radiation (shaded) and with rates registered on a carborundum evaporimeter similarly exposed. Strict linear relationships exist between evaporation from the shaded Piché evaporimeter, the Piché in a screen and the shaded carborundum evaporimeter. The latter was 1.6 times more sensitive than the shaded Piché. The shaded Piché was 1.4 times more sensitive than the screened Piché. Regression equations for the aerodynamic term (AT) in the Penman-Monteith equation were developed. AT was expressed in terms of hourly evaporation measured by a Piché in a Stevenson screen ( Epsc), an exposed, but shaded Piché ( Eps), and a carborundum evaporimeter ( Ecs). The following relationships were found: 1. (i) AT = 0.018 ( Epsc) + 0.09 (mm h −1) r = 0.95 2. (ii) AT = 0.014 ( Eps) + 0.05 (mm h −1) r = 0.96 3. (iii) AT = 0.008 ( Ecs) + 0.06 (mm h −1) r = 0.95 Estimates of maximum evapotranspiration rates, ETm, obtained by substituting these relationships for AT in the Penman-Monteith equation were then tested against lysimeter measurements of ETm. In 1985, values of the index of agreement varying between 0.94 and 0.95, and mean absolute difference varying from 0.10 to 0.09 mm h −1, for the three types of evaporimeters were obtained from these comparisons. In 1986, values of the corresponding test parameters were 0.92 and 0.09 mm h −1 when the carborundum evaporimeter was used. These values demonstrate the reliability of the proposed model. While the sensitivity of the evaporimeters varied greatly, no one proved more accurate than any other. The carborundum evaporimeter proved to be the most sensitive.

Water requirements for mesquite (Prosopis juliflora)

Evapotranspiration (ET) determined by different investigators, compared with values determined by the White and Troxell methods, showed that their values provided reasonable estimates and that utilizing diurnal water table fluctuations furnishes a method of computing ET with less than 100% vegetation density. Average daily maximum ET rates for June were calculated, and ratios were compared with values from other independent studies. Since ET rates, determined by White and Troxell methods, indicated plausible values of water use when compared to other independent studies, they could be considered index values useful as guidelines for water abstracted by mesquite.

Functions to Predict Optimal Irrigation Programs

Davis, Calif., programming dates and depths of irrigations are based on comparison of the estimated sequence of maximum evapotranspiration rates (ET m a x rates) throughout the season with the sequence of actual ET rates expected if no irrigation is given. The difference constitutes an expected sequence of ET deficits which carefully programmed irrigations may alleviate fully or in part. ET m a x depend on the sequence of leaf canopy development and on evaporative conditions at the site being planned for. ET actual rates (unirrigated) depend on the sequence of root system development, available soil water in storage at planting time, and growing season rainfall. Time relationships of corn leaf canopy and root system development researched at Davis, Calif., are combined with irrigation planning site measurements of climate, soil, and soil water content to estimate the sequence of ET deficits. Measurement of actual irrigation operational efficiency completes the needed information.

Evapotranspiration by Snap Beans Grown in Sand Nutrient Culture1

AbstractEvapotranspiration by snap bean plants was measured by continuously recording changes in volume of solution in a sand‐nutrient culture system. Evapotranspiration rate increased with increased plant growth up to complete leaf canopy coverage over the sand surface. Plants with a complete leaf canopy had a maximum evapotranspiration rate of 1.3 mm per hour at 1300 hour. Daily evapotranspiration had high correlations with vapor pressure deficit and pan evaporation.