Actual Soil Evaporation Research Articles

The role of soil texture on diurnal and seasonal cycles of potential evaporation over saturated bare soils – Lysimeter studies

• Diurnal and seasonal cycles of PE rate over three different saturated soil textures were assessed using a unique lysimeter. • The textures showed different PE sums, with the largest differences during daytime in spring and summer. • PE differences over textures were reproduced well by the energy balance method. • Simplifying the estimate of ground heat flux results in large deviations from measured PE. Calculating actual bare soil evaporation (E a ) based on potential evaporation (PE) is a widely followed approach in many disciplines such as hydrogeology, hydrology and agricultural sciences. The influence of soil texture on PE is rarely considered in these approaches. In this work, seasonal and diurnal cycles of PE over saturated bare soils were assessed using a unique lysimeter experiment in the Guanzhong Basin, China. The assessment was made for three different soil textures including fine sand (PE fine ), coarse sand (PE coarse ) and gravel (PE gravel ). Meteorological variables, soil heat flux and soil temperatures were measured at a high temporal resolution for more than 14 consecutive months. Potential evaporation rates over saturated bare soil showed clear differences between fine sand, coarse sand and gravel on an annual basis. PE fine was higher than PE coarse and PE gravel (7.3 % and 11.0 % respectively). The differences between measured PE rates over different surfaces were especially pronounced during daytime in spring and summer, but showed minor differences in autumn and winter. These results are quantitatively explained with detailed calculations of the surface energy balance and it is found that differences in available energy over the soil textures, related to different albedos, as well as different porosities and thermal properties for the materials (which influence soil temperatures) explain the differences. This shows that PE is different for different soil textures, which is neglected in most hydrological studies. Nevertheless, the full-form Penman-Monteith equation can reproduce the PE differences over soil textures quite well in autumn and winter, but a simplified approach to calculate the ground heat flux does not allow to reproduce PE differences between textures.

Estimating Soil Water Content and Evapotranspiration of Winter Wheat under Deficit Irrigation Based on SWAP Model

Deficit irrigation strategy is essential for sustainable agricultural development in arid regions. A two−year deficit irrigation field experiment was conducted to study the water dynamics of winter wheat under deficit irrigation in Guanzhong Plain in Northwest China. Three irrigation levels were implemented during four growth stages of winter wheat: 100%, 80% and 60% of actual evapotranspiration (ET) measured by the lysimeter with sufficient irrigation treatment (CK). The agro−hydrological model soil−water−atmosphere−plant (SWAP) was used to simulate the components of the farmland water budget. Sensitivity analysis for parameters of SWAP indicated that the saturated water content and water content shape factor n were more sensitive than the other parameters. The verification results showed that the SWAP model accurately simulated soil water content (average relative error (MRE) < 21.66%, root mean square error (RMSE) < 0.07 cm3 cm−3) and ET (R2 = 0.975, p < 0.01). Irrigation had an important impact on actual plant transpiration, but the actual soil evaporation had little change among different treatments. The average deep percolation was 14.54 mm and positively correlated with the total irrigation amount. The model established using path analysis and regression methods for estimating ET performed well (R2 = 0.727, p < 0.01). This study provided effective guidance for SWAP model parameter calibration and a convenient way to accurately estimate ET with fewer variables.

Modification of CSM-CROPGRO-Cotton model for simulating cotton growth and yield under various deficit irrigation strategies

Cotton (Gossypium hirsutum L.) is one of the main crops in Southern Xinjiang of China, and it is the major component of the regional economy. However, cotton cultivation in this region is facing severe challenges, e.g., the projected increase in the frequency of droughts and the decrease in precipitation in the future. Development and evaluation of deficit irrigation strategies in this region can potentially conserve irrigation water while maintaining cotton yields. In this study, the actual soil evaporation was introduced into the calculation process of stress factors in the Decision Support System for Agrotechnology Transfer (DSSAT) Cropping System Model (CSM-CROPGRO-Cotton) to simulate and evaluate the growth and yield of drip-irrigated cotton under mulching in response to various deficit irrigation strategies. The modified CSM-CROPGRO-Cotton model was calibrated using field data under full irrigation (100% reference evapotranspiration (ET0)) and validated by deficit irrigation data (0.4 ET0, 0.6 ET0 and 0.8 ET0 at the bud, flowering and boll stages) during the two cotton growing seasons of 2018 and 2019. The results showed that the modified model CSM-CROPGRO-Cotton simulated the growth index of cotton well under various deficit irrigation conditions. The index of agreement (d) of the simulated and measured biomass was greater than 0.845, and the coefficient of determination (R2) was greater than 0.967 (P < 0.01). Although the simulation of leaf area index was not as good as biomass, it also yielded satisfactory estimates, with d > 0.934 and R2 > 0.817 (P < 0.01). The simulated photoperiods in various deficit irrigation treatments were consistent with the observations. The averaged absolute relative error (ARE) in seed cotton yield over the deficit irrigation treatments was 2.1%, indicating high simulation accuracy. The results showed that the modified CSM-CROPGRO-Cotton model was appropriate for simulating cotton growth and yield under various water deficit strategies, which is a reasonable decision support system in managing irrigation for sustainable agriculture. The simulations also indicated that adopting deficit irrigation practices under normal weather conditions could conserve water without adversely affecting seed cotton yield, but there was a risk of increased yield loss under dry conditions.

ESTIMATION OF SOIL EVAPORATION BY SOIL HYDRAULIC FACTOR FROM BARE DUNE SAND MULCH

A method is presented that employs soil moisture measurements and lysimetric reference evapotranspiration data to estimate a soil hydraulic factor for estimating soil evaporation. Soil evaporation is a complicated process. The soil evaporation process has two main stages. In this study, soil hydraulic factor was used to estimate soil evaporation under dune sand mulch. The soil hydraulic factor was derived from small pan evaporation data and soil evaporation data from dune sand soil columns. Dune sand soil columns were saturated and gravitational water was drained. The dune sand soil columns were then covered by dry Tottori dune sand soil of 0 cm, 2 cm and 5 cm thickness. Soil evaporation from these columns was measured gravimetrically. Potential evaporation was determined from small evaporation pans placed alongside the dune sand soil columns. Water was applied to three weighing lysimeters (for measuring actual soil evaporation) in a glasshouse to the field capacity of the dune sand soil. The weighing lysimeters were also covered by the dune sand soil of 0 cm, 2 cm and 5 cm thickness and that was dried naturally. Cumulative soil evaporation from dune sand soil columns and cumulative potential evaporation from small evaporation pans was used to determine the soil hydraulic factor under the three dune sand mulch treatments. The soil hydraulic factor was then used to estimate soil evaporation under natural drying conditions in the glasshouse. Estimated cumulative soil evaporation was compared with measured cumulative soil evaporation. The soil hydraulic factor overestimated cumulative soil evaporation during the falling rate stage of soil evaporation under 0 cm and 2 cm sand mulch. Adjusting the estimation by subtracting the difference between the estimated and the measured cumulative soil evaporation at the point where the square root of cumulative potential evaporation was equal to the soil hydraulic factor from the estimates moved the values closer to the actual measured. The soil hydraulic factor overestimated cumulative soil evaporation during the whole drying period under 5 cm dune sand soil mulch. The adjustment of the estimates did not move the values as close to the actual measured as in the 2 cm mulch. Using the soil hydraulic factor gave better estimates of cumulative soil evaporation during the constant rate stage under 0 cm and 2 cm sand mulch than under 5 cm sand mulch.

Estimating actual and potential bare soil evaporation from silty pyroclastic soils: Towards improved landslide prediction

The estimation of evaporative fluxes and their effects on soil suction is assuming a prominent role in the field of interpretation and early-warning prediction of rainfall-induced landslides. Evaporation models refer essentially to sands or plastic (silty and clayey) soils. Models validated specifically for non-plastic silty pyroclastic soils, usually characterized by very high porosity, are instead unavailable. This deficit arises although silty pyroclastic covers are widely spread across the world, increasingly involved in rainfall-induced landslides and recognized showing particular hydrological behaviour. A number of questions may be raised about the issue: (i) may any evaporative models be reliably extended to silty pyroclastic soils?; (ii) what atmospheric variables need to be monitored at least to reliably predict evaporation fluxes in these soils?; and (iii) how accurate evaporation estimations are if they are referred to silty pyroclastic covers for early warning purposes? This study addresses these questions by assessing the capabilities of several simplified models in estimating evaporative (potential and actual) fluxes for silty pyroclastic soils. To this aim, a large-scale lysimeter, consisting in a silty pyroclastic layer exposed to the atmosphere and comprehensively monitored for both weather forcing and hydrological soil variables, is adopted. It provides a dataset of observations suitable to calibrate and validate the selected evaporation models. Moreover, the availability of weather observations makes it possible to define the minimum set of equipment required to attain reliable evaporation estimation. This study shows that: (i) the adoption of a literature-based calibration can produce misleading estimates of actual evaporation, whereas the model performances after a lysimeter-based recalibration are satisfactory; (ii) reducing the weather measurements can induce an overestimation of predicted fluxes up to 50%; and (iii) the investigated models quite accurately predict water out-coming fluxes while running in early warning predictions.

Water Use and Yield of Soybean under Various Irrigation Regimes and Severe Water Stress. Application of AquaCrop and SIMDualKc Models

Data relative to two soybean seasons, several irrigation scheduling treatments, including moderate and severe deficit irrigation, and rain-fed cropping were used to parameterize and assess the performance of models AquaCrop and SIMDualKc, the latter combined with the Stewart’s yield model. SIMDualKc applies the FAO56 dual crop coefficient approach for computing and partitioning evapotranspiration (ET) into actual crop transpiration (Tc act) and soil evaporation (Es), while AquaCrop uses an approach that depends on the canopy cover curve. The calibration-validations of models were performed by comparing observed and predicted soil water content (SWC) and grain yield. SIMDualKc showed good accuracy for SWC estimations, with normalized root mean square error (NRMSE) ≤ 7.6%. AquaCrop was less accurate, with NRMSE ≤ 9.2%. Differences between models regarding the water balance terms were notable, and the ET partition revealed a trend for under-estimation of Tc act by AquaCrop, mainly under severe water stress. Yield predictions with SIMDualKc-Stewart models produced NRMSE &lt; 15% while predictions with AquaCrop resulted in NRMSE ≤ 23% due to under-estimation of Tc act, particularly for water stressed treatments. Results show the appropriateness of SIMDualKc to support irrigation scheduling and assessing impacts on yield when combined with Stewart’s model.

A process-based water balance model for semi-arid ecosystems: A case study of psammophytic ecosystems in Mu Us Sandland, Inner Mongolia, China

We developed a process-based water balance model for semi-arid ecosystems (PWBSA) driven by daily meteorological data. The actual canopy transpiration and soil evaporation processes were simulated separately based on the Penman–Monteith model, while considering the energy partition with combined leaf area index (LAI) and canopy coverage, process-based stomatal conductance, root distribution, and available soil water, among other parameters. A simple bucket soil water model that considered preferential flow was used to simulate the soil water content in four soil layers (0–10cm, 10–20cm, 20–40cm, and 40–80cm). As a case study, the model was applied to the Mu Us Sandland, a temperate semi-arid region of China, using parameters obtained from field experiments of two psammophytic communities. The model validation using the observed leaf transpiration and daily soil water content measured in 2012 and 2013 showed that the PWBSA model simulated the water balance well for both semi-arid ecosystems. Energy partition was sensitive to changes in the LAI and canopy coverage, implying that the model is suitable for application to semi-arid ecosystems with patchy canopies. Based on the water balance simulation of the two psammophytic ecosystems during 1955–2013, differences in annual evapotranspiration, transpiration, and deep soil water loss were observed between the fixed and semi-fixed psammophytic ecosystems due to variations in the LAI, canopy coverage, species ecophysiological traits, root distribution, and soil properties. Compared with other ecosystems in semi-arid regions, psammophytic ecosystems may be more sensitive to changes in rainfall patterns and should receive more attention in future global climate change research.

Predicting Maize Transpiration, Water Use and Productivity for Developing Improved Supplemental Irrigation Schedules in Western Uruguay to Cope with Climate Variability

Various maize irrigation treatments including full and deficit irrigation were used to calibrate and validate the soil water balance and irrigation scheduling model SIMDualKc at Paysandú, western Uruguay. The model adopts the dual crop coefficient approach to partition actual evapotranspiration (ETc act) into actual transpiration (Tc act) and soil evaporation (Es). Low errors of estimation were obtained for simulating soil water content (Root mean square errors (RMSE) ≤ 0.014 cm3·cm−3 with calibrated parameters, and RMSE ≤ 0.023 cm3·cm−3 with default parameters). The ratio Es/Tc act ranged from 26% to 33% and Es/ETc act varied from 20% to 25%, with higher values when the crop was stressed offering less soil coverage. Due to rainfall regime, runoff and deep percolation were quite large. The Stewarts phasic model was tested and used to predict maize yield from Tc act with acceptable errors, in the range of those reported in literature. Water productivity values were high, ranging 1.39 to 2.17 kg·m−3 and 1.75 to 2.55 kg·m−3 when considering total water use and crop ET, respectively. Using a 22-year climatic data series, rainfed maize was assessed with poor results for nearly 40% of the years. Differently, alternative supplemental irrigation schedules assessed for the dry and very dry years have shown good results, particularly for mild deficit irrigation. Overall, results show appropriateness for using SIMDualKc to support the irrigation practice.

Evapotranspiration partitioning, stomatal conductance, and components of the water balance: A special case of a desert ecosystem in China

Summary Partitioning evapotranspiration (ET) into its components reveals details of the processes that underlie ecosystem hydrologic budgets and their feedback to the water cycle. We measured rates of actual evapotranspiration (ETa), canopy transpiration (Tc), soil evaporation (Eg), canopy-intercepted precipitation (EI), and patterns of stomatal conductance of the desert shrub Calligonum mongolicum in northern China to determine the water balance of this ecosystem. The ETa was 251 ± 8 mm during the growing period, while EI, Tc, and Eg accounted for 3.2%, 63.9%, and 31.3%, respectively, of total water use (256 ± 4 mm) during the growing period. In this unique ecosystem, groundwater was the main water source for plant transpiration and soil evaporation, Tc and exceeded 60% of the total annual water used by desert plants. ET was not sensitive to air temperature in this unique desert ecosystem. Partitioning ET into its components improves our understanding of the mechanisms that underlie adaptation of desert shrubs, especially the role of stomatal regulation of Tc as a determinant of ecosystem water balance.

The effect of different evapotranspiration methods on portraying soil water dynamics and ET partitioning in a semi-arid environment in Northwest China

Abstract. Different methods for assessing evapotranspiration (ET) can significantly affect the performance of land surface models in portraying soil water dynamics and ET partitioning. An accurate understanding of the impact a method has is crucial to determining the effectiveness of an irrigation scheme. Two ET methods are discussed: one is based on reference crop evapotranspiration (ET0) theory, uses leaf area index (LAI) for partitioning into soil evaporation and transpiration, and is denoted as the ETind method; the other is a one-step calculation of actual soil evaporation and potential transpiration by incorporating canopy minimum resistance and actual soil resistance into the Penman–Monteith model, and is denoted as the ETdir method. In this study, a soil water model, considering the coupled transfer of water, vapor, and heat in the soil, was used to investigate how different ET methods could affect the calculation of the soil water dynamics and ET partitioning in a crop field. Results indicate that for two different ET methods this model varied concerning the simulation of soil water content and crop evapotranspiration components, but the simulation of soil temperature agreed well with lysimeter observations, considering aerodynamic and surface resistance terms improved the ETdir method regarding simulating soil evaporation, especially after irrigation. Furthermore, the results of different crop growth scenarios indicate that the uncertainty in LAI played an important role in estimating the relative transpiration and evaporation fraction. The impact of maximum rooting depth and root growth rate on calculating ET components might increase in drying soil. The influence of maximum rooting depth was larger late in the growing season, while the influence of root growth rate dominated early in the growing season.

The role of soil moisture accounting in estimation of soil evaporation and transpiration

In this study, we explore how the differences in soil moisture accounting affect the estimation of actual soil evaporation (E) and transpiration (T). The main objective is, therefore, a comparative assessment of a vapor flux estimation method which has explicit soil moisture accounting, against a vapor flux estimation method that uses satellite observed soil moisture data. Three methods with different representations of water supply dynamics are compared: (1) ETLook, wherein E and T are estimated using Advanced Microwave Scanning Radiometer – Earth Observing System (AMSR-E) derived soil moisture data; (2) a simple evaporation transpiration scheme (SETS) that has a similar vaporization representation as ETLook but with soil moisture accounting based on the MOSAIC Land Surface Model; and (3) SETS-AMS which is similar to SETS except that the AMSR-E derived soil moisture controls the top layer mass balance. The schemes are compared on the Indus River Basin for the year 2007 at 1 km spatial resolution. The results suggest that downward soil water flux influences the estimation of E and T. This effect is especially dominant in areas with high soil moisture content. The comparative assessment reveals how lack of explicit soil moisture accounting may lead to an overestimation of E and underestimation of T, especially in irrigated areas.

Sensitivity of soil evaporation and reference evapotranspirationto climatic variables in South Korea

The quantification of evapotranspiration and soil evaporation is crucial for agricultural water management. The FAO-56 Penman-Monteith and E-DiGOR models were used to compute reference evapotranspiration (Eto) and bare soil evaporation, respectively, at 17 meteorological stations of South Korea, from 1980 to 2009. The same soil parameters were assumed for all stations in order to compare actual soil evaporation (Ea) rates jointly dominated by atmospheric evaporative demand and soil water availability, as well as the size of rainfall events. The sensitivity of Penman-Monteith type equations to the major climatic variables was determined based on 1-year dataset. The long-term mean annual precipitation and Eto calculated at selected stations over the country were 1339.7 mm and 1087.1 mm, respectively. Precipitation showed noticeable interyear fluctuations, and the annual Eto increased gradually during the study period. A strong correlation between pan evaporation (Epan) and Eto was observed (R = 0.808, P < 0.001), based on daily data of 30 years. Similarly, a significant correlation between Epan and potential soil evaporation (Ep) was existent (R = 0.622, P < 0.01). The Ep rates were lower than the Eto rates (Ep = 0.8 × Eto). The magnitude of Ea, as calculated with the model, reached a level of 63% of Ep. On the other hand, Ea accounted for 29.4% to 50.3% of the total precipitation over South Korea. Potential soil evaporation was more sensitive to net radiation, while reference evapotranspiration was mostly affected by the relative humidity. Wind speed was the less effective variable. The contribution of soil heat flux was negligible. The sensitivity of both Ep and Eto to the same climatic variables showed significant differences among seasons and locations. The aridity index ranged from 0.85 to 2.13, and all the study sites could be classified as humid areas. An aridity index of less than 1 appeared about once every 6 to 7 years, based on the station averages.

Assessing crop coefficients for Zea mays in the semi-arid Hailiutu River catchment, northwest China

To improve irrigation water-use efficiency, plant transpiration and soil evaporation in a maize (Zea mays L.) field in the Bulang sub-catchment of the Hailiutu River catchment in Northwest China were determined using in situ measurements. Crop transpiration (Tp) rates from Jul 15 to Oct 1, 2011 were measured with sap flow sensors, and soil evaporation (Ep) rates were measured with micro-lysimeters under an absence of water deficit. The two rates together gave the total evapotranspiration (ETc) of the maize field. Cumulative Tp and Ep were 245 and 85mm, accounting for 74 and 26% of total ETc (330mm), respectively. To calculate the total ETc rate of the maize field for the entire growing season, the Penman–Monteith equation combined with a single crop coefficient method (FAO-56) was used. The estimated crop coefficient (Kc) was calibrated using actual sap flow and soil evaporation data to provide accurate estimates of actual evapotranspiration. The total crop ETc of the maize field for the 2011 and 2012 growing seasons was 583 and 500mm, respectively, with a mean daily value of ∼4mmd−1. Groundwater contributed 33% of the maize ETc in 2011 (average groundwater table of 1.12m with full irrigation) and 27% in 2012 (average groundwater table of 0.89m with full irrigation). These results will improve precise planning and efficient management of irrigation for maize in this region.

Simulation of Soil Hydrological Components in Chuncheon over 30 years Using E-DiGOR Model

The hydrological components of a sandy loam soil of nearly level in Chuncheon over 30 years were computed using the E-DiGOR model. Daily simulations were carried out for each year during the period of 1980 to 2009 using standard climate data. Reference evapotranspiration and potential soil evaporation based on Penman-Montheith model were higher during May to August because of the higher atmospheric evaporative demand. Actual soil evaporation was mainly found to be a function of the amount and timing of rainfall, and presumably soil wetness in addition to atmospheric demand. Drainage was affected by rainfall and increased with a higher amount of precipitation and soil water content. Excess drainage occurred throughout rainy months (from July to September), with a peak in July. Therefore, leaching may be a serious problem in the soils all through these months. The 30-year average annual reference evapotranspiration and potential soil evaporation were 951.5 ㎜ and 714.2 ㎜, respectively. The actual evaporation from bare soil varied between 396.9-528.4 ㎜ and showed comparatively lesser inter-annual variations than drainage. Annual drainage rates below 120 cm soil depth ranged from 477.8 to 1565.9 ㎜. The long-term mean annual drainage-loss was approximately two times higher than actual soil evaporation.

Evaluation of compensated heat-pulse velocity method to determine vine transpiration using combined measurements of eddy covariance system and microlysimeters

A field experiment was carried out with the objective to evaluate the compensated heat-pulse velocity (CHPV) method used to determine vine transpiration (Tsap). The performance of the CHPV method was evaluated using daily values of residual transpiration (Tr), obtained as the difference between actual evapotranspiration (ETa) and soil evaporation (Es) (Tr=ETa−Es) measured from an eddy covariance (EC) system and microlysimeters, respectively. Data used in this study were collected over a drip-irrigated Merlot vineyard trained on a vertical shoot positioned (VSP) system during three consecutive growing seasons (2006/2007, 2007/2008 and 2008/2009). Results showed that the best agreement between Tsap and Tr was obtained using correction coefficients for a wound size of 2.4mm. The comparison between Tsap and Tr indicated that the index of agreement (d) was 0.97, and root mean square error (RMSE), mean absolute error (MAE) and mean bias error (MBE) were 0.22, 0.18 and −0.04mmday−1, respectively. Also, the sensitivity analysis of fraction of wood (FM), fraction of water (FL) and factor of thermal properties of the woody matrix (k) suggested that the changes of ±30% have a little effect in the final estimation of daily vine transpiration with variations less than 12%. Finally, major disagreements between Tr and Tsap were observed on partially cloudy days where rapid changes (on 30min time intervals) of solar radiation produced extreme values of volumetric sap flux density.

Sensitivity analysis of the evaporation module of the E-DiGOR model

Sensitivity analysis of the soil-water-evaporation module of the E-DiGOR (Evaporation and Drainage investigations at Ground of Ordinary Rainfed-areas) model is presented. The model outputs were generated using measured climatic data and soil properties. The first-order sensitivity formulas were derived to compute relative sensitivity coefficients. A change in the net solar radiation significantly affected potential evaporation from bare soils estimated by the Penman-Monteith equation. The sensitivity coefficients were positive, with a mean value of 0.82. The sensitivity of potential soil evaporation to soil heat flux was lower during the summer months and higher during the winter. The gradient of saturated vapour pressure–temperature curve increased or decreased the potential evaporation rates because of the occurrence of the gradient variable in both the numerator and denominator of the equation. Increases in the vapour pressure deficit increased the evaporation and its effect was more pronounced in the winter. In the case of aerodynamic resistance, the coefficients were constantly negative, and became more negative in the winter, which means a negative correlation between the input and output. In Aydın's equation, the dependent variable (actual soil evaporation) was initially very sensitive to a change in the water potential of a wet soil. The sensitivity decreased progressively during the drying period. The coefficients related to the absolute values of soil water potential were constantly negative, with a mean value of -0.19. The sensitivity of actual soil evaporation to air-dry water potential remained low in the wet soil and was higher in the drier soil. The sensitivity coefficient to measure the impact of potential evaporation on actual evaporation did not change during the study. According to Aydın and Uygur's equation, potential soil evaporation greatly affected the output (soil water potential) in the wet soils. In Kelvin's equation, increases in air temperature increased the absolute value of water potential at air-dryness. The sensitivity coefficients, owing to relative humidity, showed a great deal of fluctuation.

Simulation of soil water dynamics under subsurface drip irrigation from line sources

A mathematical model which describes water flow under subsurface drip lines taking into account root water uptake, evaporation of soil water from the soil surface and hysteresis in the soil water characteristic curve θ( H) is presented. The model performance in simulating soil water dynamics was evaluated by comparing the predicted soil water content values with both those of Hydrus 2D model and those of an analytical solution for a buried single strip source. Soil water distribution patterns for three soils (loamy sand, silt, silty clay loam) and two discharge rates (2 and 4 l m −1 h −1) at four different times are presented. The numerical results showed that the soil wetting pattern mainly depends on soil hydraulic properties; that at a time equal to irrigation duration decreasing the discharge rate of the line sources but maintaining the applied irrigation depth, the vertical and horizontal components of the wetting front were increased; that at a time equal to the total simulation time the discharge rate has no effect on the actual transpiration and actual soil evaporation and a small effect on deep percolation. Also the numerical results showed that when the soil evaporation is neglected the soil water is more easily taken up by the plant roots.

A soil water balance model for a rain-fed vineyard in a micro catchment based on dual crop coefficient

This paper describes a soil water balance model for a rain-fed vineyard in micro catchment water harvesting system based on dual crop coefficient to separate actual crop transpiration and soil evaporation in the Bajgah area, Fars Province, I. R. of Iran. In this model, root depth was divided into seven layers each with 20 cm thickness, and includes rainfall, run-off, actual transpiration, soil evaporation, water contribution from deeper layers and deep percolation. In this study, 10 micro catchments containing a grapevine tree with an area of 13.4 m2 were built in the study area and volumetric soil moisture contents at a depth of 120 cm and amount of soil water at depths of 0–120 cm were measured during the growing season of 1984–1987. Then, the model was run by using weather data during these years, and the volumetric soil moisture contents in each day were estimated. The daily value of upward water contribution from deeper soil layers was considered equal to 0.72 mm. The results showed that there was no significant difference between the values of estimated and measured volumetric soil moisture contents in depth of 120 cm, and amount of soil water at depths of 0–120 cm.

A model for Evaporation and Drainage investigations at Ground of Ordinary Rainfed-areas

Quantification of water losses through evaporation and drainage from bare soils in arid and semi-arid regions is very important for an effective management strategy to conserve soil water. In this study, a model for Evaporation and Drainage investigations at Ground of Ordinary Rainfed-areas (hereafter E-DiGOR) is presented. Daily potential evaporation ( E p) from bare soils was calculated using the Penman–Monteith equation with a surface resistance of zero. Actual soil evaporation ( E a) was computed according to Aydin et al. [Aydin, M., Yang, S.L., Kurt, N., Yano, T., 2005. Test of a simple model for estimating evaporation from bare soils in different environments. Ecol. Model. 182 (1), 91–105; Aydin, M., Yano, T., Evrendilek, F., Uygur, V., 2008. Implications of climate change for evaporation from bare soils in a Mediterranean environment. Environ. Monit. Assess. 140, 123–130]. Deep drainage ( D) was simply calculated by the mass balance, taking field capacity into account. In order to test the performance of the model mainly for drainage estimations, a micro-lysimeter-experiment was carried out under field conditions. The experimental terrain was nearly flat, with no appreciable slope. Estimated and measured water balance components such as actual evaporation ( R 2 = 91.4%; P < 0.01), drainage ( R 2 = 88.5%; P < 0.01) and soil water storage ( R 2 = 89.7%; P < 0.01) were in agreement. This agreement supported the model hypothesis, thus rendering the model useful in estimating soil evaporation, drainage and water storage in an interactive way with a few parameters. Once the estimated and measured data from the experiment had been compared for validation, simulations were carried out continuously for the period of 1994–2006 in a semi-arid environment of Turkey. E p rates were lower during the winter season because of the lesser evaporative demand of the atmosphere. However, E a rates were mainly found to be functions of the rainfall pattern, and presumably soil wetness in addition to atmospheric evaporative demand. D volumes below 120 cm soil depth were high during rainy months, with a peak in January. Annual E p varied between 850.6 and 909.8 mm during the period of 13 years. E a ranged from 248.0 to 392.9 mm with a mean annual value of 302.5 mm. D substantially varied inter-annually (150.5–757.4 mm) depending on the intensity and frequency of rainfall events and especially antecedent soil wetness. The next logical step in model development would be the inclusion of runoff for sloping lands.

Energy and water balance measurements for water productivity analysis in irrigated mango trees, Northeast Brazil

Crop water parameters, including actual evapotranspiration, transpiration, soil evaporation, crop coefficients, evaporative fractions, aerodynamic resistances, surface resistances and percolation fluxes were estimated in a commercial mango orchard during two growing seasons in Northeast Brazil. The actual evapotranspiration (Ea) was obtained by the eddy covariance (EC) technique, while for the reference evapotranspiration (E0); the FAO Penman– Monteith equation was applied. The energy balance closure showed a gap of 12%. For water productivity analysis the Ea was then computed with the Bowen ratio determined from the eddy covariance fluxes. The mean accumulated Ea for the two seasons was 1419 mm year � 1 , which corresponded to a daily average rate of 3.7 mm day � 1 . The mean values of the crop coefficients based on evapotranspiration (Kc) and based on transpiration (Kcb) were 0.91 and 0.73, respectively. The single layerKc was fitted with a degree days function. Twenty percent of evapotranspiration originated from direct soil evaporation. The evaporative fraction was 0.83 on average. The average relative water supply was 1.1, revealing that, in general, irrigation water supply was in good harmony with the crop water requirements. The resulting evapotranspiration deficit was 73–95 mm per season only. The mean aerodynamic resistance (ra) was 37 s m � 1 and the bulk surface resistance (rs) was 135 s m � 1 . The mean unit yield was 45 tonne ha � 1 being equivalent to a crop water productivity of 3.2 kg m � 3 when based on Ea with an economic counterpart of US$ 3.27 m � 3 . The drawback of this highly productive use of water resources is an unavoidable percolation flux of approximately 300 mm per growing season that is detrimental to the downstream environment and water users.