Agriculture In Semi-arid Regions Research Articles

Long-term effects of management history on carbon dynamics in agricultural soils in Northwest China

Human activities and climate change have substantial effects on soil ecosystems. The historical variations in soil organic carbon (SOC) in Northwest China were investigated in this paper. Using an agro-ecosystems database, a process-based model, Denitrification-Decomposition (DNDC), was used to support a regional SOC simulation. Over the 1980s and 1990s, the SOC storage (0–50 cm) reduced slightly from 1.25 PgC (pentagrams of carbon, 1 PgC = 1015 gC) and stabilized at 1.23 PgC. SOC significantly differed in Northwest China farmland both temporally and spatially. Climatic variability magnified the response of the spatially heterogeneous region to climate and greatly influenced the vegetation and soil. The SOC increase appeared in areas with a mild and humid climate in the east, including Shaanxi, Gansu, Ningxia and south Xinjiang. Management practices greatly affect SOC dynamics through straw C input, increased 46.31 % C budget and 20.52 % total SOC (TSOC) through 60 % crop residue incorporation. Reducing human disturbance to the surface soil through conservative tillage and improving soil fertility through reasonable nutrient input and water-use efficiency have important implications for C sequestration, emission reduction and soil productivity improvement in arid or semiarid agricultural farmland regions.

Estimating groundwater use patterns of perennial and seasonal crops in a Mediterranean irrigation scheme, using remote sensing

This work explores the use of satellite-based vegetation indices (VI) to study groundwater use in a semi-arid agricultural irrigated area. The objective is to obtain insight in spatial and temporal patterns and differences in groundwater usage of perennial (mainly fruit trees) and seasonal crops (mainly row vegetable crops) under varying climatic conditions. Cropping intensities of seasonal crops are derived for each sector and irrigation water applied (IWA) is calculated using VI-based (NDVI from MODIS) actual evapotranspiration estimates and local efficiency factors. Groundwater use is then derived as the residual of total IWA and surface water supplies for each sector and crop type. The results of IWA following this methodology were compared with survey-based results for two crop types. Results correlated well, but deviate most during drought period, likely due to salt leaching practices. Monthly groundwater use patterns and spatial and temporal differences during normal water availability and drought conditions are reported. On average, about 50% of irrigation water is extracted from aquifers, but during droughts this percentage increases considerably. Perennial crops show sharper increases in groundwater use under such conditions than seasonal crops. Overall, seasonal crops put more pressure on the groundwater resource than perennial crops. Our results and methodology will be useful for water resource managers, and policy makers concerned with the role of groundwater resources on the sustainability of semiarid agricultural regions.

Blue Water Demand for Sustainable Intensification

The agricultural challenge of meeting global food demand requires an increase in the level of agricultural water productivity and some increases in global water use. But many arid or semiarid agricultural regions of the world are facing declining water availability for irrigation. Examples of declining groundwater availability are seen throughout arid and semiarid areas of North America, Africa, and Asia. Relevant to water demand for sustainable intensification of agriculture, this paper touches on concepts where policy can work toward improving water productivity, including: (i) assessing crop water use and productivity, (ii) promoting cultural practices for increasing crop water productivity, (iii) improving efficiency of green water use, and (iv) protecting agricultural water supplies. Developing a paradigm of assessing crop water productivity and comparing with potential water‐limited yields is a valuable diagnostic tool leading to improved management and water use. Similarly, water footprinting is a water accounting tool that should be applied toward improvements in agricultural water productivity. These assessment tools can prioritize how and when agronomic practices and plant genetic improvements can best be employed. This paper also overviews some cultural practices that can improve water productivity in dryland, rainfed, and irrigated systems. Several examples of policies that influence the supply of water for irrigation are given. Innovative policy structures are needed that allow sharing of water among agricultural, municipal, and environmental users while also rewarding conservation and efficient use. Specific policies that are designed to protect water supplies for agricultural use are needed.

Sowing dates and crop replacements strategies to adapt climate change impact on traditional rain fed agriculture in semi arid regions: A case of North Kordofan in Sudan

Sowing dates and crop replacements strategies to adapt climate change impact on traditional rain fed agriculture in semi arid regions: A case of North Kordofan in Sudan

Optimizing Resource Use and Economics of Crop-Livestock Integration Among Small Farmers in Semiarid Regions of South Africa

The adoption of crop-livestock integration (CLI) among smallholder farmers in the developing countries is no doubt one of the solutions to food security, risk management, and poverty alleviation in sub-Saharan Africa. However, adequate assessment on the current status of CLI becomes necessary for the development and its evolution among smallholder farmers. This article presents a basic and multi-objective linear programming (LP) model to determine enterprise combinations of crop and livestock activities that maximize total gross margin (TGM) among small farms in the Eastern Cape Province. Optimization modeling results and trade-off analysis showed 122.61% increase and 58% decreases, respectively, compared with the average TGM per livestock unit (LU). Smallholder CLI farmers can optimally maximized profit, but fail to optimally maximize home consumption with profit maximization objectives at present resource level. The results further revealed that resources such as land, livestock enterprises were (loose resources) not efficiently used. Efficient and capital use and compact arrangement of crop-livestock integration are suggested instead of free access grazing practice in the study area.

Soil and groundwater quality with reference to nitrate in a semiarid agricultural region

Nitrate is a common pollutant in surface water and groundwater of agricultural areas. It is essential to monitor this pollutant in groundwater, especially when it is used for drinking purposes without treatment. The present study was carried out in an intensively irrigated area which forms a part of Nalgonda district, Andhra Pradesh, India where groundwater meets all the water needs of the rural population living in this area. The objective was to assess the spatiotemporal variation in the concentration of nitrate in groundwater and soil. Based on the analysis of 496 groundwater samples collected from 45 wells over a period of 2 years from March 2008 to January 2010 by sampling every 2 months, it was observed that groundwater in 242 km2 of the total 724 km2 area had nitrate above the maximum permissible limit of 45 mg/l for drinking purposes. Nitrate concentration in groundwater showed a positive relation with potassium, chloride, and sulfate, indicating their source from fertilizers. Reasons for the high concentration of nitrate in domestic areas were the dumping of animal wastes and leakage from septic tanks. The pH of the soil samples showed that most of the area had basic soil. Apart from pH, organic carbon, available phosphorous, available potassium, ammoniacal nitrogen, and nitrate nitrogen were also analyzed in the 97 soil samples.

Simulating water uptake of irrigated field crops from non-saline water table soils: Validation and application of the model SWAMP

Mathematical models are invaluable tools for irrigation farmers in semi-arid regions to assess their water management practices under water table conditions. The application of complex models remains a challenge, because determination of input variables and model parameters is tedious and complicated. Thus, the credibility of the Soil WAter Management Program, SWAMP, to assess on-farm management practices, as an alternative to the more complicated numerical models, was investigated. This was accomplished by determining how accurately SWAMP simulates water use of field crops from shallow water tables and how can the model be applied to assess current water management practices by farmers. To accomplish the objectives two data sets were used, i.e. data from a lysimeter experiment and from a case study conducted on an irrigated field located within the Orange-Riet Irrigation Scheme, South Africa. According to the aggregated accuracy, correlation and pattern analysis (ISWAMP), SWAMP performed well in simulating weekly evapotranspiration (ISWAMP=70%) and water tale uptake (ISWAMP=90%) of wheat, peas and maize grown on sand to sandy loam soils. SWAMP was also successful in solving the soil water balance under water table conditions at field level. This was done with easily obtainable inputs, while maximizing in situ field observations, which are vital considering that farmers cannot adopt alternative management practices if their current practices cannot be measured. Due to these strengths SWAMP should therefore be easily adopted by irrigation farmers and agricultural advisers to ensure efficient water use.

Soil Properties Affecting Rainfall Water Use Efficiency (RWUE) in Wheat Dry-Farming Lands, NW Iran

Effective use of rainfall water is a key issue in agricultural development in the arid and semi-arid regions since rainfall water is a precondition for crop production there. This study was conducted in a semi-arid agricultural region with 900 km2 in area in Hashtroud, northwest of Iran to determine the relationship between rainwater use efficiency (RWUE) and soil properties. Winter wheat yield and soil properties were determined at 108 plots (40.41 m2 in area) installed in thirty six dry-farming lands. RWUE of each plot obtained from the ratio of crop dry matter per unit of abstracted rainfall water volume (ARWV). ARWV was computed from deduction of the rainfall and runoff volume during a two-growth period. Runoff data for each land was obtained from field measurements at the plots under natural rainfalls. Analysis of rainfalls uniformity using four rain gauge stations data showed that spatial distributions of rainfalls were homogeneous in the area. The RWUE values in the lands were ranged from 0.35 kg m-3 to 1.49 kg m-3 with an average of 0.84 kg m-3. Soil properties which considerably affected either the infiltration capacity or the available water controlled the RWUE in the study area. Multi-regression analysis indicated that the RWUE significantly related to silt, organic matter and lime (R2=0.82, p < 0.001). Maintaining crop residues and incorporating with the soil can be proper techniques and sustainable strategies to improve the soil properties and enhance the RWUE in the dry-farming lands.

Essential oil and chemical compositions of wild and cultivated Thymus daenensis Celak and Thymus vulgaris L.

Thymus vulgaris L. and Thymus daenensis Celak from mint family (Lamiacae) traditionally being used as source of the essential oil and phenolic components derived from their different parts. Growth, oil yield and chemical components of T. vulgaris and T. daenensis grown in different regions in south-central of Iran were investigated. The hydro-distillated oils obtained from wild and cultivated thyme species were analyzed by GC–MS. Twenty four components in total were identified representing more than 85–98% of the oil composition, with thymol (33.9–70.3%), carvacrol (4.0–24.8%), γ-terpinene (3.9–10.4%) and p-cymene (4.8–8.6%) in T. daenensis, and thymol (35.5–44.4%), carvacrol (4.4–16.1%), γ-terpinene (10.5–11.9%) and p-cymene (8.5–16.1%) in T. vulgaris as major constitutes. Location had no significant effect on growth characteristics of the two thyme species, but the quantity and quality of their essential oils were different in different regions. T. daenensis cultivated in Saman region produced highest thymol (70.3%), whereas those cultivated in Shahrekord region produced highest oil yield (1.16%, v/w). T. daenensis wild growing produced highest carvacrol (24.8%). The results indicated that essential oils and their chemical compositions of Thymus species are strongly affected by environmental conditions and agronomic management practices. Cultivation of thyme, especially T. daenensis proved to be superior in both oil content and quality, in terms of substantial contents of phenolic monoterpenoids (thymol). Results of this study suggest that wild growing of T. daenensis in semiarid condition is not rich in essential oil and thymol, therefore farmers in semiarid regions should grow cultivated species for producing highest economic amount of extracted essential oil and thymol for pharmaceutical, therapeutic and food purposes.

Protocols and Guidelines for Field-scale Measurement of Soil Salinity Distribution with ECa-Directed Soil Sampling

Soil salinity is a spatially complex and dynamic property of soil that influences crop yields when the threshold salinity level is exceeded. The mapping and monitoring of soil salinity is necessary for reclamation, crop selection, and site-specific irrigation management of salt-affected soils in the arid and semi-arid agricultural regions of the world. Because of its spatial and temporal heterogeneity, soil salinity is difficult to map and monitor at field scales. There are various methods for characterizing soil salinity variability, but none of these approaches has been as extensively investigated and is as reliable and cost effective as apparent soil electrical conductivity (ECa) directed soil sampling. Geospatial measurements of ECa are well-suited for characterizing soil salinity spatial distribution because they are reliable, quick, and easy to take with GPS-based mobilized ECa measurement equipment. However, ECa is influenced by a variety of soil properties, which makes the measurement of soil salinity at field scale problematic. It is the goal of this review and analysis paper to provide an overview of the field-scale characterization of soil salinity distribution using ECa-directed soil sampling. Guidelines, special considerations, protocols, and strengths and limitations are presented for characterizing spatial and temporal variation in soil salinity using ECa-directed soil sampling. Original data is presented showing the critical importance of conducting ECa surveys at or near field capacity (≥70% of field capacity). A case study of a 32.4-ha field in San Jacinto, California, is provided as an example to demonstrate the effectiveness of generating soil salinity maps. Land resource managers, farmers, extension specialists, and Natural Resource Conservation Service field staff are the beneficiaries of field-scale maps of soil salinity.

Effect of irrigation systems on temporal distribution of malaria vectors in semi-arid regions

Previous research models have used climate data to explain habitat conditions of Anopheles mosquitoes transmitting malaria parasites. Although they can estimate mosquito populations with sufficient accuracy in many areas, observational data show that there is a tendency to underestimate the active growth and reproduction period of mosquitoes in semi-arid agricultural regions. In this study, a new, modified model that includes irrigation as a factor was developed to predict the active growing period of mosquitoes more precisely than the base model for ecophysiological and climatological distribution of mosquito generations (ECD-mg). Five sites with complete sets of observational data were selected in semi-arid regions of India for the comparison. The active growing period of mosquitoes determined from the modified ECD-mg model that incorporated the irrigation factor was in agreement with the observational data, whereas the active growing period was underestimated by the previous ECD-mg model that did not incorporate irrigation. This suggests that anthropogenic changes in the water supply due to extensive irrigation can encourage the growth of Anopheles mosquitoes through the alteration of the natural water balance in their habitat. In addition, it was found that the irrigation systems not only enable the active growth of mosquitoes in dry seasons but also play an important role in stabilizing the growth in rainy seasons. Consequently, the irrigation systems could lengthen the annual growing period of Anopheles mosquitoes and increase the maximum generation number of mosquitoes in semi-arid subtropical regions.

Impact of agriculture activities on a coastal aquifer in Tunisia and options for a better water management

The quality of a coastal aquifer (Korba, Tunisia) in a semi-arid agricultural region was studied by the collection and analysis of groundwater samples for their main chemical and biological parameters in relation to the SEQ evaluation system for groundwater quality. Results indicate important anthropogenic deterioration. Most of the samples were unsuitable for drinking-water production; animal drinking and irrigation with these waters will require caution. The high contamination is mainly due to the local intensive agriculture. Based on the objectives-oriented project planning method, regulatory and socio-economic instruments were extensively developed to encourage sustainable agriculture through a participatory exercise.

Integrating Cotton and Beef Production in the Texas Southern High Plains: I. Water Use and Measures of Productivity

Texas High Plains agriculture, largely dependent on water from the Ogallala aquifer for irrigation, exemplifies semiarid agricultural regions where irrigation is used at nonsustainable rates of extraction. Integrating crop and livestock systems has been suggested to conserve water and to achieve other environmental and economic goals compared with monoculture systems. From 1998 to 2008, two large‐scale systems, with three blocks in a randomized block design, compared irrigation water, productivity, chemical inputs, and specific pests of (i) a cotton (Gossypium hirsutumL.) monoculture, and (ii) an integrated three‐paddock system that included cotton in a two‐paddock rotation with grazed wheat (Triticum aestivumL.) and rye (Secale cerealeL.) and the perennial variety WW‐B. Dahl old world bluestem (OWB) [Bothriochloa bladhii(Retz) S.T. Blake] in a third paddock for grazing and seed production. All paddocks were irrigated by subsurface drip. Angus crossbred beef steers (Bos taurus; initial BW 229 kg; SD = 33 kg) grazed 185 d from January to mid‐July each year. During the 10 yr following the establishment year, cotton lint yield was similar and averaged 1370 kg ha−1for both systems. Bluestem seed yield averaged 25 kg pure live seed (PLS) ha−1. Steers gained 139 kg on pasture and 0.79 kg d−1. Per hectare, the integrated system used 25% less (P&lt; 0.001) irrigation water, 36% less N fertilizer, and fewer other chemical inputs than monoculture cotton. Integrated production systems that are less dependent on irrigation and chemical inputs appear possible while achieving goals of sustainability, fiber production, and food security.

Investment in Irrigation Systems under Precipitation Uncertainty

Efficient agricultural water management is indispensable in meeting future food demands. The European Water Framework Directive promotes several measures such as the adoption of adequate water pricing mechanisms or the promotion of water-saving irrigation technologies. We apply a stochastic dynamic programming model (SDPM) to analyze a farmer’s optimal investment strategy to adopt a water-efficient drip irrigation system or a sprinkler irrigation system under uncertainty about future production conditions, i.e. about future precipitation patterns. We assess the optimal timing to invest into either irrigation system in the planning period 2010 to 2040. We then investigate how alternative policies, (a) irrigation water pricing, and (b) equipment subsidies for drip irrigation, affect the investment strategy. We perform the analysis for the semi-arid agricultural production region Marchfeld in Austria, and use data from the bio-physical process simulation model EPIC (Environmental Policy Integrated Climate) which takes into account site and management related characteristics as well as weather parameters from a statistical climate change model. We find that investment in drip irrigation is unlikely unless subsidies for equipment cost are granted. Also water prices do not increase the probability to adopt a drip irrigation system, but rather delay the timing to invest into either irrigation system.

Time series analysis of irrigation return flow in a semi-arid agricultural region, Iran

The irrigation return-flow coefficient (IRFC) is based on rule of thumb estimations in most aquifers in Iran. The errors may be significant, and thus misleading for water resources management. We used lysimeters to more accurately predict IRFC. Five wheat and barley farms of different soil textures, which were irrigated using a border system, plus one with a sprinkler system, were selected for the installation of lysimeters. The practices at each farm were applied to the lysimeters. Irrigation return flows (IRF) were measured every other day over two growing seasons. The IRF time series displayed sharp peaks at the initial stages of the growing season and decreasing towards the end. IRFCs were classified into early and late stages. The average IRFC during the early stage was 29.9%, decreasing to 6.5% in the late stage. High IRFC values are due to the limitations of border irrigation to provide a small uniform amount of water, and over-irrigation due to low water demand during the early stage of growth. Variations in annual IRFCs of 0.4 to 38.7% are functions of total applied water, border length, management and soil texture. The early stage IRFC for the sprinkler system was small. Therefore, high early stage IRFCs can be reduced by changing from border irrigation to a sprinkler system.

Implications of deep drainage through saline clay for groundwater recharge and sustainable cropping in a semi-arid catchment, Australia

Abstract. The magnitude and timing of deep drainage and salt leaching through clay soils is a critical issue for dryland agriculture in semi-arid regions (&lt;500 mm yr−1 rainfall, potential evapotranspiration &gt;2000 mm yr−1) such as parts of Australia's Murray-Darling Basin (MDB). In this rare study, hydrogeological measurements and estimations of the historic water balance of crops grown on overlying Grey Vertosols were combined to estimate the contribution of deep drainage below crop roots to recharge and salinization of shallow groundwater. Soil sampling at two sites on the alluvial flood plain of the Lower Namoi catchment revealed significant peaks in chloride concentrations at 0.8–1.2 m depth under perennial vegetation and at 2.0–2.5 m depth under continuous cropping indicating deep drainage and salt leaching since conversion to cropping. Total salt loads of 91–229 t ha−1 NaCl equivalent were measured for perennial vegetation and cropping, with salinity to ≥ 10 m depth that was not detected by shallow soil surveys. Groundwater salinity varied spatially from 910 to 2430 mS m−1 at 21 to 37 m depth (N = 5), whereas deeper groundwater was less saline (290 mS m−1) with use restricted to livestock and rural domestic supplies in this area. The Agricultural Production Systems Simulator (APSIM) software package predicted deep drainage of 3.3–9.5 mm yr−1 (0.7–2.1% rainfall) based on site records of grain yields, rainfall, salt leaching and soil properties. Predicted deep drainage was highly episodic, dependent on rainfall and antecedent soil water content, and over a 39 yr period was restricted mainly to the record wet winter of 1998. During the study period, groundwater levels were unresponsive to major rainfall events (70 and 190 mm total), and most piezometers at about 18 m depth remained dry. In this area, at this time, recharge appears to be negligible due to low rainfall and large potential evapotranspiration, transient hydrological conditions after changes in land use and a thick clay dominated vadose zone. This is in contrast to regional groundwater modelling that assumes annual recharge of 0.5% of rainfall. Importantly, it was found that leaching from episodic deep drainage could not cause discharge of saline groundwater in the area, since the water table was several meters below the incised river bed.

Nitrogen fixation by pea and lentil green manures in a semi-arid agroecoregion: effect of planting and termination timing

Crop-fallow systems dominate many semi-arid agricultural regions despite fallow’s negative effects on soil and water quality. Annual legumes grown as a fallow-replacement crop, and terminated prior to maturity, can reduce these negative effects without substantially decreasing plant available water for the subsequent crop. Interest in growing legume green manures (LGMs) in synthetically-fertilized systems is increasing in the northern Great Plains of North America, partly due to the N-fixing capabilities of legumes; however, little is known about the effects of planting and termination time on N fixation amounts in the region. A 2-year field study was initiated in southwest Montana to determine the effects of planting time (spring or summer) and termination time (e.g. flower or pod) on the amount of N fixed by field pea (Pisum sativum cv. Arvika) and lentil (Lens culinaris cv. Richlea). Two methods, 15N natural abundance and N difference, were used to quantify N fixation, with wheat or in-crop weeds as reference plants. In 2009, N fixed by spring-planted lentil was higher by pod than flower (P = 0.03). Termination time did not affect the amount of N fixed by spring-planted pea, despite more biomass by pod than flower. In 2010, both spring-planted crops fixed more N by pod than flower (P < 0.01) and more N was fixed by spring-planted than summer-planted crops (P < 0.01). These results should prove useful to growers interested in selecting management practices that optimize N fixation of LGMs.

Assessment of strip tillage systems for maize production in semi-arid Ethiopia: Effects on grain yield, water balance and water productivity

The Maresha, the traditional Ethiopian plow, requires repeated cross-plowing which causes increased surface runoff, less infiltration and hence lower water availability to crops. The main reasons for increased surface runoff and reduced infiltration are plowing along the slope and the formation of a plow pan at shallow depths. Conservation tillage is seen as a way to alleviate these problems. The widely advocated zero-tillage, however, is not feasible for smallholder farmers in semi-arid regions of Ethiopia because of difficulties in maintaining adequate soil cover, the practice of communal grazing, and high costs of herbicides. Strip tillage systems, on the other hand, may offer a solution. This study was initiated to test strip tillage systems and to evaluate the impacts of new tillage systems on the water balance and grain yields of maize. Experiments have been conducted in a semi-arid area called Melkawoba in the central Rift Valley of Ethiopia during 2003–2005. Strip tillage systems involved cultivation along planting lines at a spacing of 0.75m using the Maresha plow followed by subsoiling along the same lines (STS) or without subsoiling (ST). Results have been compared with traditional tillage involving 3–4 overpasses with the Maresha plow (CONV). Soil moisture has been monitored to a depth of 1.8m using a Time Domain Reflectometer (TDR) while surface runoff has been measured using a specially designed rectangular trough installed at the bottom of each plot. STS resulted in the least surface runoff (Qs=18mmseason−1) and the highest grain yields (Y=2130kgha−1) followed by ST (Qs=26mmseason−1, Y=1840kgha−1) and CONV (Qs=43mmseason−1, Y=1720kgha−1) provided sowing was carried out within a week after subsoiling. Thus, STS resulted in the highest water productivity, WP=0.60kgm−3, followed by ST (WP=0.52kgm−3) and CONV (WP=0.48kgm−3). The main conclusion of the paper is that even in dry areas reasonable yields can be obtained provided moisture conservation in the root zone is guaranteed. In this regard subsoiling is essential. Moreover, it is concluded that the time between subsoiling and planting is a key factor and should not exceed one week.

Impacts of agricultural irrigation recharge on groundwater quality in a basalt aquifer system (Washington, USA): a multi-tracer approach

Irrigation in semi-arid agricultural regions can have profound effects on recharge rates and the quality of shallow groundwater. This study coupled stable isotopes (2Η, 18O), age-tracers (3H, CFCs, 14C), 87Sr/86Sr ratios, and elemental chemistry to determine the sources, residence times, and flowpaths of groundwater and agricultural contaminants (e.g. NO3–) in the Saddle Mountains Basalt Aquifer in central Washington, USA, where over 80% of the population depend on groundwater for domestic use. Results demonstrate the presence of two distinct types of water: contaminated irrigation water and pristine regional groundwater. Contaminated irrigation water has high NO3– concentrations (11–116 mg/l), 87Sr/86Sr ratios (0.70659–0.71078) within range of nitrogen-based fertilizers, detectable tritium (2.8–13.4 TU), CFC ages 20–40 years, high δ18O values (−16.9 to −13.5‰), and ∼100 percent modern 14C. Pristine regional groundwater has low NO3– concentrations (1–5 mg/l), no detectable tritium (≤0.8 TU), low δ18O values (−18.9 to −17.3‰) and 14C ages from ∼15 to 33 ky BP. Nitrogen and oxygen isotopes of NO3–, combined with high dissolved oxygen values, show that denitrification is not an important process in the organic-poor basalt aquifers resulting in transport of high NO3– irrigation water to depths greater than 40 m in less than 30 years.

An analysis of the tendency of reference evapotranspiration estimates and other climate variables during the last 45 years in Southern Spain

► Increasing temperature trends were detected for all the analyzed weather stations . ► Penman-Monteith ET o shows a positive trend in the last 45 years in Andalucia. ► Hargreaves gives the closest values to Penman-Monteith but does not detect any trend. ► Blaney-Criddle and Radiation methods provides ET o trends similar to Penman-Monteith. Climate change will have important implications in the agriculture of semi-arid regions, such as Southern Spain, where the expected warmer and drier conditions might augment crop water demand. To evaluate these effects, a data set consisting of observed daily values of air temperature, relative humidity, sunshine duration and wind speed from eight weather stations in Andalusia and covering the period 1960–2005 was used for estimating reference evapotranspiration (ET o ). ET o was calculated using five methods: the more complex Penman-Monteith FAO-56 (PM) equation, considered as a reference in this study, and four alternative methods with fewer data requirements, Hargreaves, Blaney-Criddle, Radiation and Priestley-Taylor. These methods were compared to PM with respect to ET o average values and trends. The non-parametric Mann-Kendall test was used to evaluate annual and seasonal trends in the main climate variables and ET o . Due to increases in air temperature and solar radiation, and decreases in relative humidity, statistically significant increases in PM-ET o were detected (up to 3.5 mm year −1 ). Although the Hargreaves equation provided the closest average values to PM, this method did not detect any ET o trend. On the other hand, trends found from Blaney-Criddle and Radiation ET o values were similar to those obtained from PM. In addition, after a local adjustment, these two methods gave accurate ET o average values. Therefore, Blaney-Criddle and Radiation methods have shown themselves to be the most accurate approaches for ET o determination in climate change studies, when available data provided by climate models are limited.