Hargreaves Equation Research Articles

Evaluation of Variability of Watershed Hydrological Partitioning: The Case of Mormora River, South Eastern Ethiopia

Water resources management in a watershed requires the quantification of the water potential. This is usually achieved by the estimation of the hydrological and meteorological characteristics of the watershed using observed data to carry out statistical estimates. Hydrological and meteorological data for the Mormora watershed were collected to analyze the hydrologic partitioning behavior of the watershed. The available data include a daily record of stream flow data from 1990 to 2007, a thirty years (1983-2012) monthly average precipitation at meteorological stations with in or near the watershed, and a 31 years (1981-2011) maximum and minimum temperature data at the same meteorological stations. The missed data of stream flow was in filled by simple regression method, and an outlier test, F-test, and T-test was computed for the data to check its quality. Necessary information about the geography of the area was obtained from LuLc, Soil, River, DEM, and climate map of Ethiopia, using ArcGIS software. Daily stream flow was converted to monthly scale and was partitioned to base flow and direct runoff using WHAT software. The maximum and minimum temperature data were used to estimate the potential evapotranspiration of the watershed using the 1985 Hargreaves equation. Average areal rainfall was estimated by Thiessen polygon method using ArcGIS software. Following these, the annual water balance of the watershed was analyzed using ‎water ‎balance ‎theory. The inter-annual variability of the water balance, and hydrologic indices were analyzed. The relationship between climate and landscape is also studied. The result shows that the Horton Index and Humidity Index of the watershed are relatively constant with a coefficient‎ of‎ variation, ‎Cv≤0.127.‎Finally‎conclusions‎are‎drawn,‎and‎the‎difficulties‎in‎ making hydrological and meteorological estimates in a watershed with little or no data are highlighted.

Statistical analysis of modified Hargreaves equation for precise estimation of reference evapotranspiration

There are a lot of meteorological stations around the world, like Pakistan, where all the meteorological parameters are not accessible to analyze Penman-Monteith (PM) method for the estimation of Reference Evapotranspiration (ETo ). So ablative methods such as Hargreaves Samani (HS) equation are therefore a potential solution to this problem, which required small numbers of meteorological parameters to estimate ETo . But, simplification brings imprecision and the need for regional calibration of the HS equation’s constant. In this study, fuzzy logic based calibration of the constants in the HS equation at the various climatic locations of Pakistan for the period of 1980 − 2019 is performed. The precision of HS and Modified HS (MHS) against standard PM method for the approximation of ETo was evaluated and compared the calibration process of HS equation by adjusting the coefficients of the equation gradually. The performance of the MHS is evaluated based on the statistical indicators such as Mean Absolute Error (MAE), Root Mean Square Error (RMSE), and Performance Index (PI). The statistical assessment of the study revealed that the MHS equation performed better as compared to the conventional HS equation and simultaneous adjustment of all the empirical parameters “ah”, “bh”,”ch” and “dh” was the best alternative for calibration of the HS equation. Furthermore, the change point detection analysis has been carried out on ETo (estimated by MHS) using Pettitt’s test, Buishand’s range test and Buishand’s U test. Results of the change point analysis indicated different change points from year 1980 to 2019, mainly due to the industrialization and urbanization during this time period.

Evaluation of Estimation Methods for Monthly Reference Evapotranspiration in Arid Climates

Reference evapotranspiration (ET0) plays a key role in irrigation system design as well as water management of agricultural ecosystems under irrigated and rainfed conditions. While many methods for estimating the ET0 have been developed during the past several decades, method selection essentially depends on the availability of measured climatic variables. The FAO-56PM method recommended by experts from Food and Agriculture Organization of the United Nations is widely used in agricultural and environmental research to estimate the ET0. However, it requires several climatic parameters that are not always available in developing countries, especially in arid regions. Here, we compare and evaluate the performance of 13 widely- and commonly-used equations for estimating ET0 against that predicted using the FAO-56PM model using climatic data from nine meteorological stations located in arid regions across Iran. On average, the best three methods that could be used as an alternative to the FAO-56PM equation were the Irmak (Irmak et al., 2003), Hargreaves-Samani (Hargreaves and Samani, 1985), and Hargreaves (1975) equations.

Effect of Water Irrigation Management and Nitrogen Fertilizer Sources on Water Productivity and Quality of Some Egyptian Cotton Cultivar

The experimental field was conducted at the experimental farm, Fac. of Ag., Al-Azhar U., Assuit, during two growing summer seasons of 2018 and 2019. A field experiment was designed as randomized complete block with split split-plot arrangement of treatment with three replications. The current study aims to evaluate different irrigation methods (conventional and alternative furrows) and nitrogen fertilizer Sources (urea as fast nitrogen fertilizer and ureaform as slow one) for maximizing the production of some cotton varieties (G80 and G90) as well as to define the most favorable irrigation manner that achieves the highest water productivity and high cotton yield and its quality. The results showed that alternate furrow irrigation (AFI) saves about 12 % of the actual evapotranspiration (ETa) compared to the conventional furrow irrigation (CFI). The ETa increased with urea fertilizer compared to ureaform fertilizer. The estimated evapotranspiration (ETo) values in both growing seasons followed the descending order of FAO Penman-Montithe > Turc > Hargreaves. It is clear that the Hargreaves equation calculated ETo efficiently for cotton crop growth under Assiut region circumstances. The AFI increased the crop water productivity (CWP) and irrigation water productivity (IWP) by 13 and 15%, respectively compared to the CFI. The irrigation methods during both growing seasons had significant effects on cotton seed yield, cotton lint percentage, boll weight, lint index, seed index, 2.5% span length, length uniformity ratio, fiber strength (pressly index) and fiber fineness. The AFI realized positive higher effect on cotton traits and yield quality than those under (CFI).

COMPARISON BETWEEN ESTIMATION METHODS OF REFERENCE EVAPOTRANSPIRATION IN BOM JESUS ​​DA LAPA, BA

Among the reference evapotranspiration (ETo) equations, the FAO-56 Penman-Monteith (PM FAO-56) model is considered the most accurate, but this model requires a greater amount of meteorological data. On the other hand, there are other methods that require fewer variables and have shown good precision according to the location. The objective of this work was to evaluate the efficiency of four methods for estimation of daily ETo, comparing them with FAO-56 PM equation in Bom Jesus da Lapa, Bahia, Brazil. To do so, a dataset from 2010 to 2017, acquired at the National Institute of Meteorology (INMET), was used. The models were analyzed by means of statistical indicators: Willmott’s concordance index, root mean square error (RMSE), mean bias error (BIAS), coefficient of determination “R²”, correlation coefficient “r” and coefficient of confidence “c”, in addition to the classification of the coefficient of confidence. The results obtained show that Hargreaves and Samani equation was the only method classified as “good” and is recommended. While the models of Camargo, Priestley and Taylor and Benevides and Lopes are not recommended to calculate ETo in the municipality.

Assessment of Reference Evapotranspiration by Regionally Calibrated Temperature-Based Equations

The Penman-Monteith (PM) method for estimation of reference evapotranspiration (ET0) uses numerous data that are often missing in many regions. In such circumstances, temperature-based equations can be used for estimating ET0. The primary objective of this note was to compare the calibrated temperature-based ET0 approaches with the standard PM equation using meteorological data from fifteen CLIMWAT stations located in the Pannonian Basin. The most of these locations are classified as humid. Twelve temperature-based equations were evaluated according to the PM method. The regionally calibrated Hargreaves equation (HT) gave the lowest RMSE at seven sites and the second lowest RMSE at three sites. New regionally calibrated Thornthwaite approach (TT) yielded the lowest RMSE three times and the second lowest RMSE seven times. These approaches resulted in the lowest average RMSE for all fifteen locations (0.28 and 0.30 mm day−1, respectively). The other temperature-based approaches significantly fall behind the HT and TT equations at most stations. The overall results show the advantage of the HT approach except for a few southeast stations where the TT approach is dominant. It can be concluded that the regionally calibrated temperature-based approaches (HT and TT) are the most suitable to estimate ET0 in the Pannonian Basin.

Reference Evapotranspiration by Hargreaves and Modified Hargreaves Equations under Semi-Arid Environment

The Penman-Monteith (FAO-56 PM) ETo equation is considered as the standard ETo equation to estimate reference evapotranspiration (ETo) under all the weather conditions of the world. But there are many regions of the globe where all the climatic data is not available to evaluate FAO-56 PM ETo equation. So, Hargreaves (HG) ETo equation can be used which required very small number of weather data. The HG ETo equation requires only air temperature as input data which is available at most of the weather stations of the world. But the major drawback of HG ETo equation is that it overestimates or underestimates FAO-56 PM ETo equation. So it becomes necessary to modify HG ETo Equation according to the local climatic conditions before it is being applied. The HGorg and modified HG ETo equations are assessed for ETo estimation under cold and hot semi-arid climatic conditions of Quetta and Zhob weather stations by using 9 years meteorological data of each weather station against FAO-56PM ETo equation. The original HG ETo equation overestimates FAO-56 PM ETo equation at Zhob weather station by giving percentage error of 15.82% and underestimate at Quetta weather station by giving percentage error of 35.02%. The coefficient of original HG ETo equation was modified by using a simple mathematical logic. The overestimation at Zhob weather station reduced to 1.89% and underestimation at Quetta station reduced to 0.87% by using modified HG ETo equation. The variations of original HG ETo equation with FAO-56 PM ETo equation has RMSE of 0.89 mm/day at Zhob weather station and 1.74 mm/day at Quetta weather station. The variations of modified HG ETo equation with FAO-56 PM ETo equation has RMSE of 0.27 mm/day at Zhob weather station and 0.29 mm/day at Quetta weather station.

Predictive Evapotranspiration Equations in Rain Gardens

Current stormwater control measure (SCM) design often does not include the dynamic process of evapotranspiration (ET) for vegetated systems. This study compared two reference ET equations with a three-year data set from rain garden weighing lysimeters. The outcome was a tool to incorporate ET into SCM design. The weighing lysimeters at Villanova University, located in southeastern Pennsylvania, were used to measure water budget parameters for three scenarios: sandy loam with UO, sand with an unconstricted outflow (UO), and sand with internal water storage (IWS). The two ET models explored were the ASCE Penman-Monteith equation (a robust model) and the Hargreaves equation (a simple model). Estimated ET values from these two equations, both with and without modifications for water availability and crop presence, were compared and calibrated (if modified) with observed data. Comparisons and calibrations were performed on a daily and storm basis to explore the applicability of the two ET models for continuous and storm approaches. The observed ET was 28%–52% of inflow over the continuous three-year period and 16–30 mm on a storm scale, making ET a significant part of the lysimeters’ water budget. Due to the experimental nature of the lysimeters, 12 of the 36 study months had additional simulated runoff, such that a smaller ET as a percentage of inflow was expected in the rain garden SCM’s water balance. The Hargreaves and ASCE Penman-Monteith equations without modification provided an adequate estimate for rain garden ET for all systems at the storm scale. Modifications to ET estimations produced by both equations through crop coefficients and a soil moisture extraction function provided a good model for storm-scale ET by reducing errors and increasing efficiencies for all weighing lysimeter types. Evapotranspiration estimates from both unmodified equations provided, at best, a marginally better estimate than the average observed rate for continuous daily rain garden ET. The application of crop coefficients and a soil moisture extraction function to both equations reduced errors in ET estimates and increased the equations’ predictive power (Nash-Sutcliffe efficiency) for all weighing lysimeter types. Both equations with modifications on a daily scale produced good ET estimates for the IWS system. For both equations, crop coefficients were found in an expected range for UO systems (0.3–1.5) but were high in the IWS system (1.6–2.0). Soil moisture extraction functions were not needed to calibrate the IWS equations on the storm scale. Both the Hargreaves equation and the ASCE Penman-Monteith equation provided an adequate model (especially with modifications) to incorporate ET into a design-storm approach to SCM design. Use of both predictive models on a daily scale has potential use in continuous simulation, as in most cases the ET estimations predicted by the equations provided a better estimate than the average of the observed daily ET rates.

Analysis of Reference Evapotranspiration Change in Korea by Climate Change Impact

In this study, the impact of climate change on the characteristics of evapotranspiration in Korea was assessed. The Regional Concentration Pathway (RCP) 4.5 and 8.5 scenarios were used to calculate the reference evapotranspiration for 62 stations of the Korea Meteorological Administration (KMA) and compared with the calculation results from observation data. The results of the analysis showed that the average annual evapotranspiration increases by 5.4% in the RCP 4.5 scenario and 9.5% in the RCP 8.5 scenario compared with the current one. On a monthly scale, the reference evapotranspiration for January, February, November and December is expected to increase by more than 10% compared to the current period. The ratio of annual rainfall to yearly evapotranspiration was used to examine the possibility of droughts for future periods, and the results were found to be relatively high in the RCP 8.5 scenario when compared to the RCP 4.5 scenario. The increase in the reference evapotranspiration due to climate change may lead to a decrease in the available water resources, and therefore, continuous monitoring and appropriate adaptation measures are necessary. Keywords: Reference Evapotranspiration, Climate Change, RCP Scenario, Hargreaves Equation

A Smart Irrigation Tool to Determine the Effects of ENSO on Water Requirements for Tomato Production in Mozambique

Irrigation scheduling is used by growers to determine the right amount and timing of water application. In most parts of Mozambique, 90% of the total yearly precipitation occurs from November to March. The El Niño Southern Oscillation (ENSO) phenomenon influences the climate in Mozambique and affects the water demand for crop production. The objectives of this work were to quantify the effects of ENSO phenomenon on tomato crop water requirements, and to create the AgroClimate irrigation tool (http://mz.agroclimate.org/) to assist farmers in improving irrigation management. This study was based on daily grid-based climate information from 1983 to 2016 from the Climate Forecast System Reanalysis. Daily crop evapotranspiration was calculated by Hargreaves equation and crop coefficients. This tool is available online and considers different planting dates, ENSO phases, and crop growing season lengths. Irrigation needs varied from less than 250 mm per growing cycle during winter to 550 mm during spring. Both El Niño and La Niña influenced the irrigation scheduling, especially from November to March. El Niño periods were related to increased water demand due to drier and warmer conditions, while the opposite was observed for La Niña. The ENSO information might be used to understand climate variability and improve tomato irrigation scheduling in Mozambique.

Effects of Surface and Subsurface Drip Irrigation on the Yield, Vegetative Growth and Water Productivity of Onions

Summary The objective of this study, conducted in the northern Serbian province of Vojvodina, was to analyze the effect of surface and subsurface drip irrigation (with drip lateral placement depths of 0.05 and 0.1 m) on the yield and water productivity of onions (Allium cepa L., var. ‘Holandski žuti’). The irrigation applied was scheduled on the basis of the water balance method. The daily evapotranspiration rate was computed using the reference evapotranspiration (ETo) based on the Hargreaves equation and the crop coefficient (kc). The irrigation rate was 30 mm, whereas the amount of water added by irrigation during the season was 150 mm. According to the results obtained, the onion yield under irrigated conditions was significantly higher than that under non-irrigated (control) conditions. Differences in the yield obtained using surface and subsurface irrigation were non-significant. The amounts of water used for evapotranspiration under irrigated and non-irrigated conditions were 363 mm and 220 mm, respectively. The value of the surface irrigation yield response factor (Ky) was 0.62, whereas the values of the subsurface irrigation yield response factor (Ky) were 0.61 (0.05 m) and 0.79 (0.1 m). Consequently, onions grown from sets proved moderately sensitive to water stress under regional climate conditions and could be grown without irrigation. The value of the irrigation water use efficiency (Iwue) ranged from 3.55 to 4.97 kg m−3, whereas the value of the evapotranspiration water use efficiency (ETwue) ranged from 3.72 to 5.22 kg m−3. The highest yield of onions was obtained using a drip lateral placement depth of 0.1 m, which is recommended for high-yielding onion production.

Water productivity for Egyptian clover as affected by different irrigation regimes and cultivation methods in the North Middle Nile Delta Region

A FIELD experiment was conducted in the two successive winter seasons ( 2016-17 and 2017-18) to investigate the impact of cultivation method; dry cultivation ( dry seed on dry soil) and traditional one (wet seed on wet soil) and irrigation regime; five different irrigation regimes as follows, irrigation till 5 cm above soil surface, (traditional, I1), using soil moisture depletion (I2), irrigation with 0.8 from pan evaporation (I3), irrigation with Hargreaves equation, 1981 (I4) and irrigation till 2.5 cm above soil surface (I5).The obtained results showed that decreasing amount of irrigation water from traditional method (I1) to 2.5 cm above soil surface (I5) reduced fresh yield by 5.49 and 6.75 ton fed.-1 with water saving ≈14.5% (≈ 336.0 m3fed.-1,) for dry seeds cultivation method, and the corresponding values 4.87 and 7.34 ton fed.-1 with water saving 10.75% (≈ 280.0 m3fed.-1,) for wet cultivation one in the overall seasons. Meaningfully, an average of ≈ 700-840 million m3 water could be saved at the national level (≈2.5 million fed.). Moreover, mean reduction in dry yield by 24.62 kg fed.-1, (6.35 %) and 13.00 kg fed-1 (3.51 %) could be resulted from dry cultivation method, and by 36.45 kg fed-1 (8.68 %) and 10.91 kg fed-1 (2.60 %) could be resulted from wet cultivation method in the first and second seasons respectively.The highest overall mean values of water consumptive use (CU) and consumptive use efficiency (Ecu, %) were recorded irrigation without any stress during the growing season (I1). Productivity of irrigation water (PIW kgm-3) and water productivity (WP kgm-3) for fresh yield was the highest values recorded under I5 (the lowest water consumed) for dry and wet cultivation methods.

Uticaj navodnjavanja kapanjem na prinos i evapotranspiraciju konoplje za vlakno (Cannabis sativa L.)

The experiments showing the effect of drip irrigation on yield and evapotranspiration of fibre hemp (Cannabis sativa L.) were conducted at the experimental field of the Alternative Crops Department, Institute of Field and Vegetable Crops, Novi Sad, Serbia. Irrigation was based on the water balance method. Daily evapotranspiration (ETd) was computed from the reference evapotranspiration (ETo) and crop coefficient (kc) 0.5, 0.9 and 1.1 from sowing to 3-4 pair of leaves, from 3-4 pair of leaves to appearance of male flowers and from appearance of male flowers to the end of the season, respectively. ETo was calculated using Hargreaves equation. The irrigation depth was restricted to the soil depth of 0.4 m. In other words, irrigation started when readily available water in the soil layer of 0.4 m was completely depleted by plants. The irrigation rate was 30 mm at the beginning of the season, 40 mm in the middle of the season, and the amount of water added by irrigation was 320 mm during the entire season. Irrigation significantly affected the yield of fresh stems, fresh leaves, flowers and plant height, but not stem diameter and fibre content. Water used on evapotranspiration in irrigation conditions (ETm) was 470 mm, while in non-irrigated control variant it amounted to 129 mm (ETa). These preliminary results could be used as a good platform for hemp growers in the region, in terms of optimizing the use of irrigation water.

Calibration and spatial modelling of daily ET0 in semiarid areas using Hargreaves equation

Evapotranspiration is difficult to measure and, when measured, its spatial variability is not usually taken into account. The recommended method to estimate evapotranspiration, Penman-Monteith FAO, requires variables not available in most weather stations. Simplified but less accurate methods, as Hargreaves equation, are normally used. Several approaches have been proposed to improve Hargreaves equation accuracy. In this work, 14 calibrations of the Hargreaves equation are compared. Three goodness of fit statistics were used to select the optimal, in terms of simplicity and accuracy. The best option was an annual linear regression. Its parameters were interpolated using regression-kriging combining Random Forest and Ordinary Kriging. Twelve easy to obtain ancillary variables were used as predictors. The same approach was used to interpolate Hargreaves and Penman-Monteith-FAO ET0 on a daily basis; the Hargreaves ET0 layers and the parameter layers were used to obtain calibrated ET0 estimations. To compare the spatial patterns of the three estimations the daily layers were integrated into annual layers. The results of the proposed calibration are much more similar to Penman-Monteith FAO results than those obtained with Hargreaves equation. The research was conducted in south-east Spain with 79 weather stations with data from 01/01/2003 to 31/12/2014.

Crop-coefficients of tomato as derived using monolithic weighing type lysimeter in mid hill region of Meghalaya

Crop coefficients (kc) was determined for tomato (Lycopersicon esculentum Mill.) with the help of UMS-GmBH cylindrical field lysimeter of 30 cm diameter and 120 cm deep and Penman-Monteith FAO-56 model. Eight other models viz. Modified Penman Method, Hargreaves equation, Samani-Hargreaves equation, Thornthwaite equation, Solar Radiation Method, Net Radiation Method, Blaney-Criddle Method and Radiation Method were also used for estimation of ET0­ and compared with Penman-Monteith model to find out the accuracy of prediction with limited weather parameters. Scatter plot and paired t-test were used for comparison. Out of all these models, Blaney-Criddle method, Solar and Net Radiation method were found to yield similar results as given by Penman-Monteith model. The values of crop evapo-transpiration (ETc) were varying from 2.54 mm d-1 to 6.70 mm d-1. The crop-coefficients (kc) for three growth stages of tomato viz., initial, mid and maturity were found to be 0.55, 1.07 and 0.78, respectively.

Comparison of different reference evapotranspiration (ET0) models and determination of crop-coefficients of french bean (Phesiolus vulgaris.) in mid hill region of Meghalaya

The water requirement and crop evapotranspiration (ETc)is determined using reference evapotranspiration (ET0) and crop-coefficient (kc). Numerous models are available for estimation of ET0,among which Penman-Monteith (FAO-56) model is considered to be the most accurate and universally acceptable. In present study eight models for estimation of ET0viz. Modified Penman method, Hargreaves equation, Samani-Hargreaves equation, Thornthwaite equation, Solar radiation method, Net radiation method, Blaney-Criddle method and Radiation method were compared with Penman-Monteith model to find out the accuracy of prediction with limited weather parameters. Among these, Net radiation and Solar radiation models were found to yield relatively closer values. A field experiment was also conducted withfrench bean (Phesiolus vulgaris.) crop in UMS-GmbH cylindrical field lysimeter of 30 cm diameter and 120 cm deep for determination of crop coefficient taking Penman-Monteith FAO-56 model as the base model for ET0 estimation. The values of ETc as determined in field lysimeter varied from 3.80 mm-d-1 to 5.89 mm-d-1. The kc for initial, mid and maturity were found to be 0.45, 1.01 and 0.39, respectively.

Impact of ET0 method on the simulation of historical and future crop yields: a case study of millet growth in Senegal

ABSTRACTThe reference evapotranspiration (ET0) is an integrated climatic variable from which many crop models derive simulated crop yields. In most of these models, different equations are parameterized leaving the choice of the equation to the user. However, the impact of the choice of the ET0 equations on crop yield prediction has been little studied.The present study proposes a sensitivity analysis of the impact of the choice of the ET0 equation on simulated millet yields using SARRA‐H crop model over 12 Senegalese stations representative of the Sudano‐Sahelian climate conditions of West Africa.Priestley‐Taylor, a modified Priestley‐Taylor and Hargreaves equations lead to simulated yields up to 19% than those calculated using the Penman‐Monteith equation. Despite high biases in wind speed, among the tested methods, the Penman‐Monteith method remains the most robust to derive ET0 and yield over the major part of Senegal, Hargreaves equation being more appropriated under dry climates. The choice of ET0 formulation introduces uncertainties representing 8% of baseline yield regardless of precipitation changes; for wet conditions these uncertainties approach 30% of the overall climate change impact. The choice of ET0 equation is increasingly important, with local temperature changes out to 4 °C, while extreme changes above 6 °C depend less on the ET0 equation.

Modified Hargreaves equation for estimation of weekly reference evapotranspiration for Junagadh

Modified Hargreaves equation for estimation of weekly reference evapotranspiration for Junagadh

Water balance in the complex mountainous terrain of Bhutan and linkages to land use

Abstract Study Region: Bhutan Study Focus: Located in the Himalayas with elevation ranging 100–7550 m and with an area equivalent to Switzerland, Bhutan has great biodiversity despite its small area. A monsoon-dominated climate causes generally wet summer and dry winter. Bhutan is highly dependent of climatic conditions for its developmental activities. Using multiple regression analysis we have established models to predict the evapotranspiration (ETo) and water balance and test the linkage to vegetation and land cover using meteorological data from 70 weather stations across Bhutan. Temperature-based ETo equations were evaluated in reference to the Penman-Monteith (PM) method and a calibrated Hargreaves (H) equation was used for computing the ETo. New Hydrological Insights for the Region. The calibrated Hargreaves equation gave good estimates of average daily ETo comparable to the PM ETo. The spatial variation in PM ETo is linked to variation in sunshine hours in summer and temperature in other seasons. Seasonal and annual ETo was mainly affected by elevation and latitude, which is linked to temperature and sunshine duration. Precipitation and water balance correlated positively with the Southern Oscillation Index (SOI) while ETo correlated negatively. Our models for predicting ETo and water balances performed clearly better than the global CRU gridded data for Bhutan. A positive water balance is found in broadleaf forest areas and small or negative water balance for coniferous forests.

Variabilidad espacio-temporal de las sequías en la cuenca Pacífico Norte de México (1961-2010)

En este trabajo se analiza la variabilidad espacio-temporal de las sequías en la Cuenca Pacífico Norte de México entre 1961 y 2010, mediante dos índices de sequía: el Índice de Precipitación Estandarizada (SPI) y el Índice de Precipitación Evapotranspiración Estandarizada (SPEI). Para ello se han utilizado 48 estaciones meteorológicas con datos de precipitación, temperatura máxima y mínima mensual, de ellas en 22 estaciones también se dispuso de datos de evaporación media medida mediante evaporímetros Piché. La evapotranspiración de referencia, necesaria para calcular el SPEI, se obtuvo mediante la ecuación de Hargreaves. Se ha comprobado que la sequía es un fenómeno frecuente en la Cuenca del Pacífico Norte y que independientemente del impacto del posible estrés relacionado con la evapotranspiración, se registran episodios numerosos, algunos de ellos de gran intensidad, asociados a anomalías de precipitación durante largos periodos. Los resultados muestran que los principales episodios de sequía ocurrieron en las décadas de 1980 y 2000, aunque existe una notable variabilidad espacial a lo largo de la región. Se discute el posible impacto de la demanda de agua por parte de la atmósfera en la evolución de la sequía observada con dichos índices, y sus implicaciones para una adecuada valoración del riesgo.