Calculation Of Evapotranspiration Research Articles

Using energy balance data for assessing evapotranspiration and crop coefficients in a Mediterranean vineyard

Improving water use efficiency is a key element of water management in irrigated viticulture, especially in arid or semi-arid areas. In this study, the micrometeorological technique “Eddy Covariance” was used to directly quantify the crop evapotranspiration (ET) and to analyze the complex relationships between evapotranspiration, energy fluxes, and meteorological conditions. Both observed Direct measurements (DIR) of latent heat flux (LE) and observed from the residual of the energy balance (REB) equation were used for crop evapotranspiration calculations. Observed crop coefficients (K cms) were then determined using the standardized reference evapotranspiration (ETo) equation for short canopies. In addition, linear approximations from observations were used to model the seasonal trend lines for crop coefficients and K cs values were parameterized by first identifying the beginning and end of each growth stage. The modeled K cs values were used to predict daily ET from ETo measurements and compared with values from literature. The daily observed DIR ET values (ETdo) were lower than REB ET (ETro) during periods with precipitation, but they were similar during dry periods, which implies that energy balance closure is better when the surface is drier. Comparisons between modeled ET and crop ET estimated using K c values from best agreement was observed between the modeled REB and FAO 56 and the local K c values provided by the Regional Agency ARPAS showed good agreement with observed ET (from DIR and REB data) than the FAO 56 ones. The study confirmed that the availability of locally driven K c could be relevant to quantify the crop water requirement and represents the starting point for a sustainable management of water resources.

Evaluation of evapotranspiration and deep percolation under mulched drip irrigation in an oasis of Tarim basin, China

Summary Mulched drip irrigation for cotton field is an effective measure for the utilization of saline water, and the regulation of soil water and salt. However, the reasonable methods for quantifying actual evapotranspiration (ET) and deep percolation of recharge to groundwater are still not very well understood, which restricts the accurate regulation of soil water and salt for cotton growth in oasis. In this paper, a set of experiments of mulched drip irrigation with brackish water were conducted in a typical arid region of Tarim basin in southern Xinjiang, China. The irrigation events were recorded, and ET and fluctuations of groundwater table were carefully measured for two consecutive irrigation periods of flowering and bolling stages. A group of upscaling conversion methods were used to quantify the ET, in which canopy structure was considered to estimate the transpiration from leaf scale to a unit of field scale. The groundwater table had a significant response to the irrigation events, thus the deep percolation was estimated using water-table fluctuation method (WTF). Results showed that during the two irrigation events of flowering and bolling stages, the total ET was 31.1 mm with the soil surface evaporation of only 0.4 mm. The total percolation of recharge to groundwater was 48.2 mm which contributed to the groundwater run-off of 22.1 mm. Transpiration of 30.7 mm accounted for 98.6% of the total ET of 31.1 mm and 34.3% of the irrigation water of 90.6 mm. Compared with transpiration, the deep percolation accounted for 53.2% of irrigation water, indicating a serious excessive irrigation that recharged to groundwater. Soil salt budget showed that the salt leached into groundwater was 1.56 times of the input from brackish irrigation water and fertilization during the two irrigation periods. Even for the irrigation practice with brackish water, the accumulated salt of soil profile could also be leached out under large amount of irrigation water (e.g. 90.6 mm for the two irrigation periods, 10 days). However, the waste of enormous water which for instance occupied 53.2% of the irrigation water in this study was not conducive to the sustainable utilization of water resources in the arid oasis. Furthermore, the methods introduced in this paper for ET and deep percolation calculation of cotton filed could be used to quantify the oasis hydrologic cycle of micro-irrigation, to gain a better understanding of the ecological process.

Modified method of aerodynamic resistance calculation and its application to potential evapotranspiration estimation

Abstract The aim of this study was to present and validate an alternative evapotranspiration calculation procedure that includes specific expression for the aerodynamic resistance. Calculated daily potential evapotranspiration totals were compared to the results of FAO56 procedure application and to the results of measurements taken with a precision weighing lysimeter permanently grown with irrigated, short grass. For the examination period from March 17 through October 31, 2011, it was found that daily potential evapotranspiration estimates obtained by both calculation procedures fitted well to the lysimeter measurements. Potential evapotranspiration daily totals calculated with the use of the proposed aerodynamic resistance calculation procedure gave better results for days with higher evapotranspiration, compared to the FAO56 method. The most important is that the approach based on the proposed alternative aerodynamic resistance could be effectively used even for a wide variety of crops, because it is not limited to any particular crop.

Spatial Uniformity in Sensitivity Coefficient of Reference ET in the Texas High Plains

<abstract> <b><sc>Abstract. </sc></b>Evapotranspiration (ET) is a major component of the agricultural water budget and accurate ET estimations are essential for effective irrigation scheduling. Therefore, it is an important aspect of production agriculture and agricultural research. Potential crop ET can be calculated by multiplying reference ET by an appropriate crop coefficient. Reference ET can be estimated from meteorological data obtained from weather stations sited on a reference crop surface, e.g., grass or alfalfa. However, weather station sensor inaccuracies may propagate measurement errors into the reference ET calculations. A sensitivity analysis of weather variables used in calculating ET (air temperature, wind speed, solar radiation, and relative humidity) can be used to determine relative effects of data inaccuracies on ET estimates, and conversely, the sensor accuracy levels needed for acceptably accurate ET estimates. In this study, sensitivity analyses of weather variables on daily reference ET were conducted using hourly data (1995-2008) from eight weather stations managed by the Texas High Plains ET network in the Texas Panhandle. Sensitivity coefficients were calculated for each of the weather variables. Comparison of the sensitivity coefficients for any one station indicated that reference ET was most sensitive to wind speed followed by air temperature. Comparison of sensitivity coefficients for any one weather variable (e.g., wind speed) across the region showed no significant differences in the sensitivity of reference ET to those variables by location. This indicates that the existing North-South air temperature gradient and variability in local wind circulation patterns have limited influence on the sensitivity of reference ET to individual weather variables. This indicated that a uniform maintenance protocol should be sufficient for weather stations located throughout the Texas High Plains.

Is scale really a challenge in evapotranspiration estimation? A multi-scale study in the Heihe oasis using thermal remote sensing and the three-temperature model

Many remotely sensed evapotranspiration (ET) calculations are based on algorithms that include an aerodynamic resistance-related parameter, which is significantly affected by heterogeneity and could cause the ET to be underestimated as the scale increases, and it is challenging to verify the ET estimation because of the lack of a bridge between satellite-based remote sensing and ground-based measurements. To address this issue, an approach using a “thermal infrared remote sensing+three-temperature model” is proposed and applied under multi-scale conditions. It is hypothesized that good consistency exists between multi-scale aggregations and ET estimates using the proposed method because it does not include aerodynamic resistance. The mothed was verified in the Zhangye Oasis during the first thematic experiment launched under the HiWATER project using extensive eddy correlation (EC) tower observations (a total of 16 EC towers) and multi-scale thermal infrared datasets, including aerial photos (3m, as a bridge between the ground and satellite measurements), ETM+ (60-m resolution), ASTER (90m), and MODIS (1000m). The results show that good agreement is obtained between the EC-measured ET and that using remote sensing techniques under multi-scale conditions. The determination coefficients (R2) between the measured and estimated latent heat flux (LE) values were 0.72, 0.81, 0.61, and 0.53 for the aerial photos, ETM+, ASTER and MODIS, respectively. The respective mean absolute errors (MAEs) were 63.64, 56.56, 59.32, and 80.34Wm−2, and the mean absolute percent errors (MAPEs) were 12.00, 11.18, 10.78, and 14.60%, respectively. However, the estimated ET became slightly overestimated as the sensor resolution decreased. In addition, the variability of the heterogeneous landscape represented by the range of LE values and the standard deviation decreased gradually as the sensor resolution decreased. Aggregation (up-scaling) of ET from higher-resolution images could better represent the heterogeneity over land surfaces than the original (unaggregated) ET maps, and there is high consistency between multi-scale aggregation and estimation, with a minimum R2 of 0.76 and a maximum root-mean-square error (RMSE) of 64.80Wm−2. The proposed methodology is concluded to be a feasible method for estimating ET under multi-scale conditions over heterogeneous landscapes.

Symbolic Regression Equations for Calculating Daily Reference Evapotranspiration with the Same Input to Hargreaves-Samani in Arid China

To present an alternative simple equation for reference evapotranspiration (ET o) estimation, the symbolic regression (SR) method was applied to establish equations with the same inputs to simple Hargreaves-Samani (HS) equation in arid China. For most of the equations derived by SR method for each station, their performance increased with an increase in the equation complex index (CI). The most precise equation performed well although it was always complex and greatly varied in form. On the other hand, the simplest one was uniform in equation structure and performed slightly better than the HS equation for all the five stations, and sometimes better than the local calibrated HS equation. A trade-off equation was selected with almost the same equation form for all the five stations and low CI index. The site-specific trade-off equation performed better than the simplest one and the locally calibrated HS equation. Then parameters in the trade-off equation were unified for all the five stations, it did not perform as good as the site-specific one, but performed better than the HS equation and unified local calibrated HS equation. Thus, the SR method is suitable to determine both the site-specific and the unified equation among stations for daily ET o calculation in arid regions.

Mapping evapotranspiration coefficients in the Paris metropolitan area

Global changes involves adaptation and updated knowledge about the systems affected by these changes. Due to innovation and population growth also, the land cover pattern is exposed to modify its components, both natural and artificial, with implication for the several fields of Earth Sciences. Here, we present the spatial distribution of the land cover evapotranspiration coefficients (K clc ) in one of the most dynamic and important territory from Europe, in Paris metropolitan area. Specified crop coefficients were analysed for different crops and vegetative areas, but also the evapotranspiration coefficients related to the urban areas, bare soils and rocks, open water and rivers were presented in the present paper. The investigation has included the review and the adapted analysis of the land cover coefficients taken from the specific literature. Using the ArcGIS environment, the spatial distribution of the land cover coefficients related to four seasons (initial season, mid-season, end-season, and cold season) was carried out. The main results indicate maximum values up to 1.6 of the K clc in the mid-season, for the areas occupied by the broad-leaved forest. In the same season, the K clc high values of 1.35 and 1.5 spread in the areas covered by complex cultivation patterns, non-irrigated arable land, respective by mixed forest. The initial season presents values of K clc that range from 0.1 to 1.3, while in the end season the land cover coefficients range from 0.2 to 1.5. The cold season indicates the lower values in major part of the Paris metropolitan area with few locations, in the South and East sides of the study area, where the coefficient reaches value 1. This work highlights the variation of the crop coefficients, but also of different land cover types in the metropolitan area of Paris with implications for agriculture management, hydrogeologists, and climatologists. Further investigations will include evapotranspiration calculations and climate effects assessment on the water resources in this area

Assessment of foliage clumping effects on evapotranspiration estimates in forested ecosystems

In forested ecosystems, the aggregation of leaves into different spatial structures at the canopy, branch and shoot scales leads to a non-random spatial distribution of foliar elements. However, effect of foliar aggregation or clumping on the estimation of terrestrial evapotranspiration (ET) is not yet well understood. To evaluate the effect of foliar clumping, the process-based Boreal Ecosystem Productivity Simulator (BEPS) is used to simulate ET at eight flux tower sites in North American forests. BEPS separates a canopy into sunlit and shaded leaf groups in the calculation of canopy-level ET and clumping affects this separation. Three cases are simulated with BEPS, and the modeled ET values are compared with flux tower measurements: Case I serves as a baseline, in which LAI and clumping index at the sites are considered. In this case, BEPS can explain 43–75% the variance of measured ET at these sites. For Case II, the LAI is considered but clumping is ignored; resulting in an overestimation of annual ET at all sites by ∼5% at the most clumped sites. In Case III, when effective LAI is used (i.e. clumping is not considered), the site-averaged mean annual ET is underestimated by 11.5%, with the largest underestimation found at the site with most clumping (CA-DF49; 19.1%). In both Case II and Case III, the more clumped a canopy is, the larger bias is found in the ET estimation (p<0.001). The estimated biases are robust to the errors in key driving variables and model parameters. When LAI is derived from optical measurements on the ground and from satellite platforms without considering the effect of clumping, it can cause substantial underestimation of ET.These results demonstrate the need for considering foliage clumping in process-based ET modeling, with potential biases having large implications for carbon cycle modeling, water budget calculations and understanding and predicting ecosystem responses to future climatic scenarios.

Calibration of the Hargreaves–Samani method for the calculation of reference evapotranspiration in different Köppen climate classes

The Penman–Monteith equation (FAO-56) is accepted as the standard model for estimating reference evapotranspiration (ETo). However, the major obstacle to using FAO-56 widely is that it requires numerous climatic data. The Hargreaves–Samani (HS) method is frequently used for the calculation of ETo since it is based on measurements of daily minimum and maximum air temperature alone. Those are commonly recorded at many meteorological stations throughout the world. It is the objective of this paper to evaluate the quality of HS and calibrate the coefficients of this method for different climates as represented by the Köppen classification. Estimated values are compared with Penman–Monteith ETo values in terms of the coefficient of efficiency Ceff as well as the root mean square error, the mean absolute error and the Bayes information criterion. The Penman–Monteith equation for ETo (FAO-56) is based on physics and known to provide best estimates of ETo. The results of our work show that the correlation between long-term monthly means of HS and FAO-56 can be improved significantly by introducing climate-class specific coefficients.

Modelling regional variability of irrigation requirements due to climate change in Northern Germany

The question whether global climate change invalidates the efficiency of established land use practice cannot be answered without systemic considerations on a region specific basis. In this context plant water availability and irrigation requirements, respectively, were investigated in Northern Germany. The regions under investigation – Diepholz, Uelzen, Fläming and Oder-Spree – represent a climatic gradient with increasing continentality from West to East. Besides regional climatic variation and climate change, soil conditions and crop management differ on the regional scale. In the model regions, temporal seasonal droughts influence crop success already today, but on different levels of intensity depending mainly on climate conditions. By linking soil water holding capacities, crop management data and calculations of evapotranspiration and precipitation from the climate change scenario RCP 8.5 irrigation requirements for maintaining crop productivity were estimated for the years 1991 to 2070. Results suggest that water requirement for crop irrigation is likely to increase with considerable regional variation. For some of the regions, irrigation requirements might increase to such an extent that the established regional agricultural practice might be hard to retain. Where water availability is limited, agricultural practice, like management and cultivated crop spectrum, has to be changed to deal with the new challenges.

Evaluation of the Penman-Monteith (FAO 56 PM) Method for Calculating Reference Evapotranspiration Using Limited Data

Reference evapotranspiration (ETo) is often calculated using the Penman-Monteith (FAO 56 PM; Allen et al 1998) method, which requires data on temperature, relative humidity, wind speed, and solar radiation. But in high-mountain environments, such as the Andean paramo, meteorological monitoring is limited and high-quality data are scarce. Therefore, the FAO 56 PM equation can be applied only through the use of an alternative method suggested by the same authors that substitutes estimates for missing data. This study evaluated whether the FAO 56 PM method for estimating missing data can be effectively used for paramo landscapes in the high Andes of southern Ecuador. Our investigation was based on data from 2 automatic weather stations at elevations of 3780 m and 3979 m. We found that using estimated wind speed data has no major effect on calculated ETo but that if solar radiation data are estimated, ETo calculations may be erroneous by as much as 24%; if relative humidity data are estimated, the error may b...

Capability of a regional climate model to simulate climate variables requested for water balance computation: a case study over northeastern France

This paper documents the capability of the ARW/WRF regional climate model to regionalize near-surface atmospheric variables at high resolution (8 km) over Burgundy (northeastern France) from daily to interannual timescales. To that purpose, a 20-year continuous simulation (1989–2008) was carried out. The WRF model driven by ERA-Interim reanalyses was compared to in situ observations and a mesoscale atmospheric analyses system (SAFRAN) for five near-surface variables: precipitation, air temperature, wind speed, relative humidity and solar radiation, the last four variables being used for the calculation of potential evapotranspiration (ET0). Results show a significant improvement upon ERA-Interim. This is due to a good skill of the model to reproduce the spatial distribution for all weather variables, in spite of a slight over-estimation of precipitation amounts mostly during the summer convective season, and wind speed during winter. As compared to the Meteo-France observations, WRF also improves upon SAFRAN analyses, which partly fail at showing realistic spatial distributions for wind speed, relative humidity and solar radiation—the latter being strongly underestimated. The SAFRAN ET0 is thus highly under-estimated too. WRF ET0 is in better agreement with observations. In order to evaluate WRF’s capability to simulate a reliable ET0, the water balance of thirty Douglas-fir stands was computed using a process-based model. Three soil water deficit indexes corresponding to the sum of the daily deviations between the relative extractible water and a critical value of 40 % below which the low soil water content affects tree growth, were calculated using the nearest weather station, SAFRAN analyses weather data, or by merging observation and WRF weather variables. Correlations between Douglas-fir growth and the three estimated soil water deficit indexes show similar results. These results showed through the ET0 estimation and the relation between mean annual SWDI and Douglas-fir growth index that the main difficulties of the WRF model to simulate soil water deficit is mainly attributable to its precipitation biases. In contrast, the low discrepancies between WRF and observations for air temperature, wind speed, relative humidity and solar radiation make then usable for the water balance and ET0 computation.

The Effects of Cropping Methods on Growth, Crop Index and Yield Response to Water of Rice (Oryza sativa L.) in Rainfed Agriculture

In this study, four combinations of crops: rice (C), rice-maize (MCS1), rice-cassava (MCS2) and rice-maize-cassava (MCS3) with 3 m  3 m each plots at two field areas—Saptosari and Tanjungsari were observed. Both field areas are located in Gunungkidul district, South-Central of Java Island, with that 93% at those areas are 185 m to 500 m above sea level and high proportion of multiple cropping systems (MCS). The aim of this study was to investigate the effect of different cropping method on growth, crop index and yield response to water of rice in rainfed agriculture. Mathematical models were developed to describe rice growth. The rice height was followed monomolecular function and the number of tillers followed exponential polynomial function. Crop index was calculated from climate data during plant growth phase. And yield response to water was calculated from actual evapotranspiration (ETa) and the maximum evapotranspiration (ETm). The results showed that the height of rice was not significantly different between each combination (P > 0.05). Number of tillers was also not significant (P > 0.05). However, monoculture treatment had more number of tillers than rice in MCS. Crop index of rice at Saptosari was higher than at Tanjungsari. Based on the calculation of evapotranspiration (ET), water deficit at initial was less than at mid-season (ETa 0.05).

Development of a regional specific crop coefficient (Kc) for castor (Ricinus communis L.) in Florida, USA by using the sap flow method

Although castor (Ricinus communis L.) oil has high commercial value for industrial uses as well as a feedstock for biodiesel, the United States has relied on imported castor oil since 1972. In Florida, the crop has been considered as an alternative crop to the more traditional agronomic row crops and would likely be produced under center pivot or other irrigation systems to maximize yield and fit with existing cropping systems. To develop an appropriate Kc value for the central Florida region, research was conducted at the University of Florida’s Plant Science Research and Education Unit in Citra, FL. Season-long crop transpiration was measured for the castor cultivar ‘Brigham’ in 2011 and 2012 and soil evaporation rates were quantified in 2012. Crop transpiration was obtained by sap flow analysis when the plots had reached complete canopy cover, were fully irrigated, and when daily solar radiation levels were at or above a historical radiation average to accurately reflect optimum growing conditions. Daily average plant transpiration (57,199 and 56,560Lha−1 in 2011 and 2012, respectively) were predominantly greater than daily soil evaporation (38Lha−1 in 2012), and as a result, the season long plant transpiration was the main factor included for the calculation of crop evapotranspiration (ETc). The ETc was evaluated against the reference evapotranspiration (ETo) from site specific meteorological data to calculate daily Kc values. The 10-day Kc averages were compared to published Kc values from the Food and Agriculture Organization (FAO) for castor. The crop developmental timeframes in this study were roughly aligned with the FAO timeframes when making these comparisons. The mid-season Kc values were similar to the FAO reference (1.19, Florida vs. 1.15 FAO ref.), but the late season Kc values for castor in Florida were higher as compared to the FAO reference (1.50 vs. 0.55, respectively). The increase in water use during this late season developmental period could be explained by a lack of physiological decline in this sub-tropical climate and continued irrigation application for this trial during the latter part of the season. These results provide appropriate Kc values for central Florida and will aid growers in irrigation decisions when producing castor in this region.

A catchment as a simple dynamical system: Characterization by the streamflow component approach

Summary The simple dynamical system approach was implemented to analyze, explain and simulate streamflow fluxes in diverse seasonal hydrological conditions within the forested Padež stream catchment in SW Slovenia. The catchment is characterized by the flushing, torrential hydrological response conditioned by the flysch geological settings of a low hydraulic conductivity. Consequently, the streamflow formation is not controlled solely by the deeper subsurface catchment storage but is also strongly influenced by the rainfall–runoff that bypasses the deeper subsurface part of the total catchment storage. Therefore, fast component of the streamflow is identified using two-component hydrograph separation; the component recession behavior is described by a separate sensitivity function and used in a simple model to simulate the streamflow. According to the simulation results, the Padež stream catchment behaves primarily like a deeper subsurface storage-dependent system during most of the hydrological conditions. When rainfall intensities increase (rainfall intensities close to 10 mm/h or higher), triggering of the secondary streamflow formation mechanism described by separate, bypassing flow sensitivity function becomes evident and causes fast hydrograph formation with steeply rising and falling limbs. To be able to implement the modeling concept for streamflow predictions, the rainfall losses, most likely associated with interception losses not covered under the potential evapotranspiration calculation, would have to be more thoroughly analyzed through rainfall interception measurements. Our study shows the possible way that two hydrological concepts, the streamflow recession analysis and the two-component hydrograph separation based on relatively easily measurable tracers, such as electrical conductivity, could be combined for analyzing streamflow fluxes.

Climate and water availability indicators in Canada: Challenges and a way forward. Part III – Future scenarios

Following from Parts I and II of this series (this issue), some common approaches for developing and assessing future scenarios of water availability are reviewed, along with recent case studies of Canadian watersheds. The results of future changes in drought related to climate change are influenced by the choice of indicators. For the Standardized Precipitation Index (SPI), small changes in drought frequency and severity are projected over southern Canada, reflecting the influence of increases in future annual precipitation only. However, assessments using the Palmer Drought Severity Index (PDSI) reveal dramatic increases in the potential for future droughts since this indicator incorporates the combined influences of higher temperatures, soil moisture capacity and precipitation to estimate evapotranspiration. Regarding projected changes in runoff, watersheds in British Columbia tend to show increases in annual and winter runoff. Some watersheds show projected decreases in summer runoff. In the southern Prairies, most watersheds show projected decreases in annual and summer runoff. In Ontario and Quebec, results are mixed. Lake levels in the Great Lakes are projected to decline under most scenarios, but results differ between regional and global climate model-based scenarios due to differences in how lake evaporation is calculated. In New Brunswick, Labrador and northern Quebec, streamflow is projected to increase. Uncertainties in future projections emerge due to differences between climate scenario generating methods, and between hydrologic models used for the assessments. This paper concludes with some thoughts on addressing important research questions related to future scenarios of water availability in Canada. For scenario-based assessments, hydrologic model inter-comparisons might yield some useful insights into uncertainties in model structure that affect evaporation, evapotranspiration and snowmelt calculations. Scenarios developed for assessments should include both future climate and projected land use/cover changes and, where necessary, integration of potential reduction in glacier volume. Finally, as scenarios from regional climate models become more readily available, there may be more opportunity to explore how runoff projections could be applied to basin-scale routing models.

Simple equation for estimating actual evapotranspiration using heat units for wheat in arid regions

Estimation of actual evapotranspiration (ETa) is an important part of agricultural water management in local and regional water balance studies. At the field scale, ETa is important in irrigation planning and scheduling and is an integral part of field management decision support tools. The conventional approach of estimating actual evapotranspiration is difficult and needs more calculations and extensive data on soil-plant-atmosphere. A field experiment was conducted to simplify the measurement and calculation of actual evapotranspiration by using thermal units (heat units) for spring wheat crop under trickle irrigation system in sandy soil. Two irrigation methods were applied; the first one (A) using the crop evapotranspiration (ETc) that depends on whether parameters, and the second (B) is the depletion from field capacity which dependent on soil parameters. Three varieties of wheat namely Sids12, Misr2 and Gemmeza10 were cultivated on sand soil and treatments arranged in complete randomized block design with three replicates.Application of treatment (B) resulted in highly significant increase in yield production of Gemmeza10 and Misr2 as compared to treatment (A). Grain yield of different wheat varieties grown under treatment (B) could be ranked in the following descending order: Misr2 > Gemmeza10 > Sids12. While under treatment (A) it could be arranged in the following descending order: Misr2 > Sids12 > Gemmeza10. On the other hand, the overall means indicated non-significant difference between all wheat verities. The highest values of water and irrigation use efficiency as well as heat use efficiency were obtained with treatment (B). The equation used in the present study is available to estimate ETa under arid climate with drip irrigation system.

AS INCERTEZAS ASSOCIADAS ÀS CONDIÇÕES CLIMÁTICAS OBTIDAS PELO MODELO ETA CPTEC/HADCM3: AVALIAÇÃO COMPARATIVA ENTRE OS DADOS SIMULADOS E OBSERVADOS DE PRECIPITAÇÃO, EVAPOTRANSPIRAÇÃO E VAZÃO NA BACIA HIDROGRÁFICA DO RIO IJUÍ, BRASIL.

Este estudo tem como objetivo avaliar as condições climáticas obtidas pelo modelo Eta CPTEC/HadCM3, conduzido por quatro membros do modelo global HadCM3 (CNTRL, LOW, MID e HIGH), na bacia do rio Ijuí, RS. O período de controle utilizado foi de 1961 a 1975, visando a avaliação dos cenários climáticos e de vazão entre 1976 e 1990. O trabalho foi estruturado em cinco etapas: interpolação espacial dos dados climáticos; correção das séries climáticas simuladas; cálculo da evapotranspiração de referência; simulação hidrológica da vazão mensal; comparação entre as condições simuladas e as observadas de precipitação, evapotranspiração e vazão. Embora os métodos de correção utilizados para eliminar as tendências das séries climáticas simuladas tenham originado cenários bem distintos, nenhum método apresentou desempenho superior a outro em todos os critérios analisados. Tanto no caso das precipitações, quanto das vazões resultantes do processo de modelagem hidrológica, as diferenças entre os valores simulados com base no modelo Eta e os valores observados atingiram erros algumas vezes superiores a 20%. Portanto, deve-se considerar que estas incertezas serão reproduzidas em cenários futuros, quanto à análise dos efeitos das mudanças climáticas na disponibilidade hídrica.

Water extraction variability in the banana root zone affects the reliability of water balance

Spatial and temporal variability of soil water extraction from the root zone affect soil water balance determination. The number of sensors installed in the root zone in studies addressing water balance is still set arbitrarily. This study provided an investigation of the water extraction process by banana (Musa spp.) roots by (i) determining the variability of water extraction from the banana tree root zone, (ii) detecting differences in the estimation of evapotranspiration (ET) by the soil water balance method when the number of soil profiles monitored in the roots zone varies, (iii) and; determining the minimum number of Time Domain Reflectometry (TDR) probes needed to obtain ET precision and accuracy similar to that determined by a drainage lysimeter. The field experiment was conducted in Cruz das Almas, in the state of Bahia, Brazil, where a drainage lysimeter was installed on a banana plantation. The water extraction in the banana root zone was quantified by the water content variations monitored in 72 points by TDRs, with measurements at 15-min intervals. The variability of water extraction in the banana root zone was medium to high. The range of variability affects the reliability of the crop evapotranspiration calculation by the soil water balance method. To prevent an overestimation of banana evapotranspiration, the water extraction in the soil profile must be monitored with at least 16 TDR probes installed at a minimum distance of 0.9 m and a minimum depth of 0.7 m.

Estimation of Potential Evapotranspiration at Botanga Irrigation Scheme in the Northern Region of Ghana

Climatic parameters such as temperature, rainfall, wind speed, relative humidity and sunshine hours were used to assess reference evapotranspiration and potential crop evapotranspiration of selected crops. The Penmann Monteith method for calculation of Reference Evapotranspiration, which has been incorporated in CROPWAT software, was used. Test crops included rice, tomato and pepper, which were part of the major crops cultivated in the Botanga irrigation scheme. Research findings indicated that temperature (r = 0.653) played a crucial role in assessing reference evapotranspiration and potential evapotranspiration. Temperature was lowest during the months from July to September due to lower solar radiation and higher rainfall. The potential crop evapotranspiration of rice, tomato and pepper were 697, 533 and 427 mm/season respectively. Rice growers will require more water for irrigation to ensure higher production due to its high evapotranspiration rate. DOI: http://dx.doi.org/10.5755/j01.erem.70.4.7752