Increasing water productivity of irrigated crops under limited water supply at field scale

Borkhar district is located in an arid to semi-arid region in Iran and regularly faces widespread drought. Given current water scarcity, the limited available water should be used as efficient and productive as possible. To explore on-farm strategies which result in higher economic gains and water productivity (WP), a physically based agrohydrological model, Soil Water Atmosphere Plant (SWAP), was calibrated and validated using intensive measured data at eight selected farmer fields (wheat, fodder maize, sunflower and sugar beet) in the Borkhar district, Iran during the agricultural year 2004-2005. The WP values for the main crops were computed using the SWAP simulated water balance components, i.e. transpiration T, evapotranspiration ET, irrigation I, and the marketable yield Y_M in terms in terms of Y_MT⁻¹, Y_M ET⁻¹ and Y_M I⁻¹.The average WP, expressed as $ T⁻¹ (US $ m⁻³) was 0.19 for wheat, 0.5 for fodder maize, 0.06 for sunflower and 0.38 for sugar beet. This indicated that fodder maize provides the highest economic benefit in the Borkhar irrigation district. Soil evaporation caused the average WP values, expressed as Y_M ET⁻¹ (kg m⁻³), to be significantly lower than the average WP, expressed as Y_MT⁻¹, i.e. about 27% for wheat, 11% for fodder maize, 12% for sunflower and 0.18 for sugar beet. Furthermore, due to percolation from root zone and stored moisture content in the root zone, the average WP values, expressed as Y_MI⁻¹ (kg m⁻³), had a 24-42% reduction as compared with WP, expressed as Y_M ET⁻¹.The results indicated that during the limited water supply period, on-farm strategies like deficit irrigation scheduling and reduction of the cultivated area can result in higher economic gains. Improved irrigation practices in terms of irrigation timing and amount, increased WP in terms of Y_MI⁻¹ (kg m⁻³) by a factor of 1.5 for wheat and maize, 1.3 for sunflower and 1.1 for sugar beet. Under water shortage conditions, reduction of the cultivated area yielded higher water productivity values as compared to deficit irrigation.     

Spectral vegetation indices for benchmarking water productivity of irrigated cotton and sugarbeet crops

Irrigation performance and water productivity can be benchmarked if estimates of spatially distributed yield and crop water use are available. A commonly used method to estimate crop evapotranspiration in irrigated areas is to multiply reference evapotranspiration values by appropriate crop coefficients. This study evaluated convenient ways to derive such coefficients using multispectral vegetation indices obtained by remote sensing. Detailed ground radiometric measurements were taken in small plots perpendicular to the crop rows to obtain canopy reflectance values. Ancillary measurements of green ground cover, plant height, leaf area index and biomass were taken in the cropped strip covered by the radiometer field-of-view. The results were up-scaled using 10 Landsat-5 and 1 Landsat-7 images. Crop measurements and ground radiometry were made at the time of Landsat overpass on two commercial fields, one grown with sugarbeet and the other with cotton. Crop height and ground cover were determined weekly in these two fields, three additional sugarbeet fields and one additional cotton field. The ground and satellite observations of canopy reflectance yielded similar results. Two vegetation indices, the normalized difference vegetation index (NDVI) and the soil adjusted vegetation index (SAVI) were evaluated. Both indices described the crop growth well, but SAVI was used in further evaluations because it could be conveniently related to both ground cover and the basal crop coefficient using a simple model. Based on these findings, crop water use variability was analyzed in a large sample of sugarbeet and cotton fields, within a homogeneous irrigation scheme in Southern Spain. The yield versus evapotranspiration data points were highly scattered for both cotton and sugarbeet. The yield values obtained from the sugarbeet fields and cotton fields were substantially lower than values predicted by a linear yield function, and close to a curvilinear yield function, respectively. Evapotranspired water productivity varied in the cotton fields from 0.3 to 0.78 kg m⁻³, and in the sugarbeet fields from 7.15 to 14.8 kg m⁻³.     

Water use efficiency of irrigated wheat in the Tarai region of India

Experiments were conducted during the winter seasons of 1983–1984 and 1984–1985 to identify suitable irrigation regimes s for wheat grown after rice in soils with naturally fluctuating shallow water table (SWT) at a depth of 0.4 to 0.9 m and medium water table (MWT) at a depth of 0.8 to 1.3 m. Based on physiological stages, the crop was subjected to six irrigation regimes viz., rainfed (I₀); irrigation only at crown root initiation (I₁); at only crown root initiation and milk (I₂); at crown root initiation, maximum tillering and milk (I₃); at crown root initiation, maximum tillering, flowering and milk (I₄); and at crown root initiation, maximum tillering, flowering milk and dough (I₅). Tube-well water with an EC <0.4 dsm^–1 was used for irrigation. Based on 166 mm effective precipitation during the cropping season, 1983–1984 was designated as a wet year and 1984–1985 with 51 mm as a dry year. The change in profile soil water content W (depletion) in the wet year was less (23%) under SWT and 10% under MWT as compared to the dry year. The ground water contribution (GWC) to evapotranspiration (ET) was 58% under SWT and 42% under MWT conditions in both the years. The GWC in the wet year was 20% under SWT and 23% under MWT. Of the total net water use (NWU), about 85% was ET and 15% drainage losses. The NWU was highest (641 and 586 mm) in I₅ under SWT and MWT conditions, respectively, but not the yield (5069 kg ha^–1). Compared to I₅, NWU in I₂ treatment decreased by 10% in the wet and 25% in the dry year. A similar trend was observed in the I₃ treatment under MWT condition. However, there was no statistically significant difference between yields of the I₁ to I₅ treatments of either water table depth during the wet year. This was also true during the dry year for the I₂ to I₅ treatments. Under SWT, in I₂, the grain yield was 5130 kg ha^–1 and under I₃ regime, 5200 kg ha^–1. Under MWT in I₃, the yield was 5188 kg ha^–1 and under I₄ regime, 5218 kg ha^–1. Thus it appears that in the Tarai region where the water table remains shallow (<0.9 m) and medium (<1.3 m) for most of the wheat growing season applications of more than 120 and 180 mm irrigation under SWT and MWT conditions, respectively were not necessary. Irrigation given only at crown root initiation and milk stages under shallow water table conditions, and at crown root initiation, maximum tillering and milk stages under medium water table conditions, appears to be as effective as frequent irrigations.     

Spatial and temporal variability of nitrate in irrigated salad crops

The objective of this study was to analyze the spatial and seasonal variations in NO₃ ^–-N concentration in soil samples and solution samplers and the N leaching of an irrigated crop cultivated intensively in the Mediterranean zone. Although much information is available from controlled field experiments concerning N concentration and its spatial variability, quantitative estimates of nitrate fluxes under normal farming conditions and when the field is directly managed by farmers are rare. This is particularly true for gardening crops in the Mediterranean zone, where high evapotranspiration rates lead to intensive irrigation and may be responsible for N leaching. A field experiment was conducted in the Departement du Gard under agricultural conditions. Salads (Cichorium endivia, Lactuca sativa) were planted in three consecutive periods. The field was irrigated with sprinklers. Local measurements with a neutron probe were made at two sites (row, interrow), and an experimental plot (95 m×25 m) was surveyed at 36 points located on a 10 m×10 m equilateral grid to analyze the spatial variability of water and NO₃ ^–-N balances. To analyze the basic statistical properties of our sampling scheme, random fields of soil concentration were simulated with the turning-bands method. Sampling strategy simulations indicated that when a spatial structure exists, sampling according to a regular grid was more efficient than a purely random sampling strategy. Global trends indicated high spatial variability for nitrate leaching with differences between periods of different irrigation intensity (97 kg ha^–1 NO₃ ^–-N leaching during the spring and summer, and 199 kg ha^–1 NO₃ ^–-N leaching during autumn and winter). Leaching caused temporal variations in the spatial distributions of NO₃ ^–-N. The origin of the spatial variability of N leaching was explained by first, the variability in NO₃ ^–-N concentration in the soil profile, and second, by spatial variability in irrigation. Furthermore, the spatial distribution of the NO₃ ^–-N concentration was time dependent, and NO₃ ^–-N spatial distributions became independent after approximately 2 or 3 months under our conditions. Our results show that better management of irrigation and fertilizer in spring and summer may reduce N leaching and, thus, improve ground water quality. Received: 15 March 1996     

Modeling the water transport and nitrogen dynamics in irrigated salad crops

The use of N fertilizers in agriculture is crucial, and agricultural techniques need to be implemented that improve significantly N fertilizer management by reducing downward movements of solutes through the soil. To achieve this, it is essential to develop and test models against experimental conditions in order to improve them and to make sure that they can be applied to a broad range of soil and climatic conditions. A field experiment was carried out in the French department of Gard. The soil was a clay loam (26.7% clay, 44.7% fine and coarse silt, and 28.6% fine and coarse sand). Salad vegetables (Cichorium endivia, Lactuca sativa) were cultivated during two consecutive periods (spring and summer crops). The crops were planted on punched and permeable plastic mulching bands. The field was irrigated with a sprinkler watering system. Local measurements were made combining a neutron probe, tensiometers, and ceramic porous cups to estimate NO₃-N concentrations. The model is one-dimensional and is based on Richards' equation for describing saturated-unsaturated water flow in soil. At the soil surface, the model is designed to handle flux-type or imposed-pressure boundary conditions. In addition, provision is made in the model, for example, to account for a mulch plastic sheet that limits evaporation. The model accounts for heat transport by diffusion and by convection, while the modeling of the displacement of nitrate and ammonium in the soil is based on the convection-dispersion equation. Nitrate uptake by the crop is modeled assuming Michaelis-Menten kinetics. Nitrogen cycle modeling accounts for the following major transformations: mineralization of organic matter, nitrification of ammonium, and denitrification. The results showed that the overall trend of the water potential in the soil profile was correctly described during the crop seasons. Mineralization was high for the spring crop (4.7 kg NO₃-N day^–1 ha^–1), whereas the other sink components, such as root uptake, drainage, and denitrification, were smaller (1.9, 1.4, and 0.2 kg NO₃-N day^–1 ha^–1, respectively). For the summer crop, intensive denitrification was found in the soil layer at 0.15–0.90 m (5.7 kg NO₃-N day^–1 ha^–1), while the mineralization was always an important component (9.2 kg NO₃-N day^–1 ha^–1) and the sink terms were 1.7 and 1.7 kg NO₃-N day^–1 ha^–1 for root uptake and drainage, respectively. Similar high denitrification rates were found in the literature under intensive irrigated field conditions. Received: 25 October 1995     

Expansion of urban area and wastewater irrigated rice area in Hyderabad,India

The goal of this study was to investigate land use changes in urban and peri-urban Hyderabad and their influence on wastewater irrigated rice using Landsat ETM + data and spectral matching techniques. The main source of irrigation water is the Musi River, which collects a large volume of wastewater and stormwater while running through the city. From 1989 to 2002, the wastewater irrigated area along the Musi River increased from 5,213 to 8,939 ha with concurrent expansion of the city boundaries from 22,690 to 42,813 ha and also decreased barren lands and range lands from 86,899 to 66,616 ha. Opportunistic shifts in land use, especially related to wastewater irrigated agriculture, were seen as a response to the demand for fresh vegetables and easy access to markets, exploited mainly by migrant populations. While wastewater irrigated agriculture contributes to income security of marginal groups, it also supplements the food basket of many city dwellers. Landsat ETM + data and advanced methods such as spectral matching techniques are ideal for quantifying urban expansion and associated land use changes, and are useful for urban planners and decision makers alike.     

Increasing productivity in irrigated agriculture: Agronomic constraints and hydrological realities

Irrigation is widely criticised as a profligate and wasteful user of water, especially in watershort areas. Improvements to irrigation management are proposed as a way of increasing agricultural production and reducing the demand for water. The terminology for this debate is often flawed, failing to clarify the actual disposition of water used in irrigation into evaporation, transpiration, and return flows that may, depending on local conditions, be recoverable. Once the various flows are properly identified, the existing literature suggests that the scope for saving consumptive use of water through advanced irrigation technologies is often limited. Further, the interactions between evaporation and transpiration, and transpiration and crop yield are, once reasonable levels of agricultural practices are in place, largely linear—so that increases in yield are directly and linearly correlated with increases in the consumption of water. Opportunities to improve the performance of irrigation systems undoubtedly exist, but are increasingly difficult to achieve, and rarely of the magnitude suggested in popular debate.     

Review of measured crop water productivity values for irrigated wheat,rice, cotton and maize

The great challenge of the agricultural sector is to produce more food from less water, which can be achieved by increasing Crop Water Productivity (CWP). Based on a review of 84 literature sources with results of experiments not older than 25 years, it was found that the ranges of CWP of wheat, rice, cotton and maize exceed in all cases those reported by FAO earlier. Globally measured average CWP values per unit water depletion are 1.09, 1.09, 0.65, 0.23 and 1.80 kg m⁻³ for wheat, rice, cotton_seed, cotton_lint and maize, respectively. The range of CWP is very large (wheat, 0.6–1.7 kg m⁻³; rice, 0.6–1.6 kg m⁻³; cotton_seed, 0.41–0.95 kg m⁻³; cotton_lint, 0.14–0.33 kg m⁻³ and maize, 1.1–2.7 kg m⁻³) and thus offers tremendous opportunities for maintaining or increasing agricultural production with 20–40% less water resources. The variability of CWP can be ascribed to: (i) climate; (ii) irrigation water management and (iii) soil (nutrient) management, among others. The vapour pressure deficit is inversely related to CWP. Vapour pressure deficit decreases with latitude, and thus favourable areas for water wise irrigated agriculture are located at the higher latitudes. The most outstanding conclusion is that CWP can be increased significantly if irrigation is reduced and crop water deficit is intendently induced.     

Water balance of centre pivot irrigated pasture in northern Victoria, Australia

The irrigated dairy industry in Australia depends on pasture as a low-cost source of fodder for milk production. The industry is under increasing pressure to use limited water resources more efficiently. Pasture is commonly irrigated using border-check but there is growing interest amongst dairy irrigators to explore the potential for overhead sprinklers to save water and/or increase productivity. This paper reports on a detailed water balance study that evaluated the effectiveness of centre pivot irrigation for pasture production. The study was conducted between 2004/2005 and 2005/2006 on a commercial dairy farm in the Shepparton Irrigation Region in northern Victoria. More than 90% of supplied water (irrigation plus rainfall) was utilized for pasture growth. Deep drainage of respectively 90 and 93 mm was recorded for the two observation seasons. During the 2004/2005 season, deep drainage resulted from large unseasonal summer rainfall events. Over the 2005/2006 season, deep drainage resulted from excess irrigation. The cumulative pasture dry matter (DM) production was 15.5 and 11.3 tonnes DM ha⁻¹ for the two irrigation seasons, with an agronomic water use efficiency (WUE) of 16 and 12 kg DM ha⁻¹ mm⁻¹ respectively. The farmer's intuitive irrigation scheduling was found to be very effective; the pattern of irrigation application closely matched measured pasture water use, prevented water stress and resulted in high irrigation efficiency.     

Evaluation of the Watermark sensor for use with drip irrigated vegetable crops

The Watermark 200SS sensor was evaluated for the measurement of soil matric potential (SMP) with drip-irrigated vegetable crops. Pepper and melon crops were grown sequentially during autumn-winter and spring-summer, in a sandy loam soil in a greenhouse. Ranges of SMP were generated by applying three different irrigation treatments — 100, 50 and 0% of crop water requirements, during two treatment periods (16 December 2002–7 January 2003; 20 January–10 February 2003) in pepper and one treatment period (26 May–6 June 2003) in melon. Watermark sensors and tensiometers were positioned, at identical distances from irrigation emitters, at 10 cm soil depth, with four replicate sensors for each measurement. Electrical resistance from Watermark sensors and SMP from tensiometers were recorded at 30-min intervals. An in-situ calibration equation was derived using data from the first pepper treatment period. For data in the three treatment periods, SMP was calculated from Watermark electrical resistance using the in-situ, Thomson and Armstrong (in Appl Eng Agric 3:186–189 1987), Shock et al. (1998) and Allen (2000) calibration equations. Additionally, the Thomson and Armstrong (in Appl Eng Agric 3:186–189 1987) and Shock et al. (1998) equations were re-parameterised with the SOLVER^® function of Microsoft Excel 2000^® using data from the first pepper treatment period. Watermark-derived SMP, for each equation, were compared with tensiometer-measured SMP, for <-10, ?10 to ?30, ?30 to ?50 and ?50 to ?80 kPa ranges, using visual analysis, and relative root mean square error (RRMSE) and mean difference (Md) values. In rapidly drying soil, the Watermark-derived SMP responded considerably more slowly to continual drying and to drying between irrigations, regardless of the calibration equation used. Otherwise, the Watermark sensor was able to provide an accurate indication of SMP, depending on the calibration equation. The in-situ and re-parameterised equations were accurate for the conditions in which they were derived/re-parameterised. However, as the growing conditions increasingly differed from those original conditions, these equations lost their advantage compared to the two published equations, suggesting that they are not robust approaches. The Thomson and Armstrong (in Appl Eng Agric 3:186–189 1987) equation generally provided an accurate indication of SMP at >?30 kPa, measuring to ?2.5 kPa. Where the soil was not drying rapidly, the Shock et al. (1998) equation generally provided an accurate indication of SMP at ?30 to ?80 kPa. The use of dynamic data (collected every 30 min) compared to static data (collected only at 6 a.m.) did not influence the evaluation of calibration equations. This study suggested that the Watermark sensor can provide an accurate indication of SMP provided that a suitable calibration equation is derived/verified for the specific cropping conditions, and that the performance characteristics of the sensor are considered.     

A global benchmark map of water productivity for rainfed and irrigated wheat

The growing pressure on fresh water resources demands that agriculture becomes more productive with its current water use. Increasing water productivity is an often cited solution, though the current levels of water productivity are not systematically mapped. A global map of water productivity helps to identify where water resources are productively used, and identifies places where improvements are possible. The WATPRO water productivity model for wheat, using remote sensing data products as input, was applied at a global scale with global data sets of the NDVI and surface albedo to benchmark water productivity of wheat for the beginning of this millennium. Time profiles of the NDVI were used to determine the time frame from crop establishment to harvest on a pixel basis, which was considered the modelling period. It was found that water productivity varies from approximately 0.2 to 1.8 kg of harvestable wheat per cubic metre of water consumed. From the 10 largest producers of wheat, France and Germany score the highest country average water productivity of 1.42 and 1.35 kg m⁻³, respectively. The results were compared with modelling information by Liu et al. (2007) who applied the GEPIC model at a global scale to map water productivity, and by Chapagain and Hoekstra (2004) who used FAO statistics to determine water productivity per country. A comparison with Liu et al. showed a good correlation for most countries, but the correlation with the results by Chapagain and Hoekstra was less obvious. The global patterns of the water productivity map were compared with global data sets of precipitation and reference evapotranspiration to determine the impact of climate and of water availability reflected by precipitation. It appears that the highest levels of water productivity are to be expected in temperate climates with high precipitation. Due to its non-linear relationship with precipitation, it is expected that large gains in water productivity can be made with in situ rain water harvesting or supplemental irrigation in dry areas with low seasonal precipitation. A full understanding of the spatial patterns by country or river basin will support decisions on where to invest and what measures to take to make agriculture more water productive.     

The effect of rice straw mulch on evapotranspiration, transpiration and soil evaporation of irrigated wheat in Punjab, India

Soil evaporation (Es) is considered to be a non-productive component of evapotranspiration (ET). So, measures which moderate Es may influence the amount of water available for transpiration (T), the productive component of ET. Field experiments investigating the effects of rice straw mulch on components of the water balance of irrigated wheat were conducted during 2006-2007 and 2007-2008 in Punjab, India, on a clay loam soil. Daily Es was measured using mini-lysimeters, and total seasonal ET was estimated as the missing term in the water balance equation. Mulch lowered total Es over the crop growth season by 35 and 40 mm in relatively high and low rainfall years, respectively. Much of this “saved water” was partitioned into T, which increased by 30 and 37 mm in the high and low rainfall years, respectively. As a result, total ET was not affected by mulch in either year. In both years, there was a trend for higher biomass production and grain yield with mulch, but with significant differences only in 2006-2007. Transpiration efficiency (TE) with respect to grain yield was 18.8-19.1 kg ha⁻¹ mm⁻¹ in 2006-2007, and 14.6-16.4 kg ha⁻¹ mm⁻¹ in 2007-2008. While wheat grown in the presence of mulch tended to lower TE, this was only significant in 2007-2008. The results suggest that while mulching of well-irrigated wheat reduces Es, it does not “save” water because the crop compensates by increased T and reduced TE.     

Water-use efficiency of irrigated winter maize under cool weather conditions of India

Field experiments were conducted during 1993/94 and 1994/95 in the sub-humid tropic environment of northern India to identify suitable irrigation schedule(s) for winter maize (December to May). Based on plant growth stages, viz. knee-high, tasselling, flowering, silking, grain-filling and dough, which occurred, respectively, at 55, 75, 95, 105, 125 and 145 days after planting, the crop was subjected to six irrigation treatments, which were: no irrigation (I₀); irrigation given at all the growth stages (I₁); irrigation missed at knee-high (I₂); at knee-high and dough (I₃); at knee-high, flowering and grain-filling (I₄); and at knee-high, flowering, silking and dough stages (I₅). The change in profile soil water content, (W (depletion) of the entire crop-growing season was found to be in the order I₀ >I₅ >I₄ >I₃ >I₂ >I₁. Of the total net water use (NWU), about 87% was evapotranspiration and 13% deep percolation losses. The NWU was highest (472 and 431 mm) under I₁ and lowest (223 and 240 mm) under the I₀ treatment during the two cropping seasons. Compared to I₁, NWU in I₃ decreased by 23% and 12.3% and in I₄ by 33.8% and 24.2% in the two cropping seasons. However, there was no statistically significant difference (at LSD, P=0.05) between yields of the I₁ to I₄ treatments during either year. The NWU was found to be in the order I₁ >I₂ >I₃ >I₄ >I₅ >I₀, whereas the water-use efficiency (WUE) based on NWU was found to be in the reverse order: I₅ >I₄ >I₃ >I₀ >I₂ >I₁. Maximum yield (5.14 t ha^-1) with WUE of 1.39 kg m^-3 was obtained under the I₃ treatment. However, optimum yield (4.91 t ha^-1) with high WUE of 1.54 kg m^-3 was under I₄. Accordingly, irrigation applications greater than 240 mm did not provide additional yield of winter maize. Frequent irrigations (I₁) proved detrimental to grain yield of winter maize in the northern Indian plains, especially under cool weather conditions, where minimum temperature (6°C) can be accompanied by occasional frost.     

Rising water table: A threat to sustainable agriculture in an irrigated semi-arid region of Haryana, India

The sustainability of the rice-wheat cropping system in an irrigated semi-arid area of Haryana State (India) is under threat due to the continuous rise in the poor quality groundwater table, which is caused by the geo-hydrological condition and poor irrigation water management. About 500,000 ha in the State are waterlogged and unproductive and the size of the waterlogged area is increasing. We analyse the hydrology and estimate seasonal net groundwater recharge in the study area. Rainfall is quite variable, particularly in the monsoon season, and the mean monthly reference evapotranspiration shows a high inter-annual variation, with values between 2.45 and 8.47 mm/day in December and May. Groundwater recharge analysis during the study period (1989-2008) reveals that percolation from irrigated fields is the main recharge component with 57% contribution to the total recharge. An annual groundwater table rise of 0.137 m has been estimated for the study area. As the water table has been rising continuously, suitable water management strategies such as increasing groundwater abstraction by installing more tubewells, using the groundwater conjunctively with good quality canal water, changes in cropping patterns, adoption of salt tolerant crops, changes in water-pricing policy, and matching water supply more closely with demand, are suggested to bring the water table down to a safe limit and to prevent further rising of the water table.     

Operational aspects of satellite-based energy balance models for irrigated crops in the semi-arid U.S.

SEBAL and METRIC remote sensing energy-balance based evapotranspiration (ET) models have been applied in the western United States. ET predicted by the models was compared to lysimeter-measured ET in agricultural settings. The ET comparison studies showed that the ET estimated by the remote sensing models corresponded well with lysimeter-measured ET for agricultural crops in the semi-arid climates. Sensitivity analyses on impacts of atmospheric correction for surface temperature and albedo showed that the internal calibration procedures incorporated in the models helped compensate for errors in temperature and albedo estimation. A repeatability test by two totally independent model applications using different images, operators and weather datasets showed that seasonal estimations by the models have high repeatability (i.e. stable results over ranges in satellite image timing, operator preferences and weather datasets). These results imply that the SEBAL/METRIC remote sensing models have a high potential for successful ET estimation in the semi-arid United States.     

Water requirement of drip irrigated tomatoes grown in greenhouse in tropical environment

Four different levels of drip fertigated irrigation equivalent to 100, 75, 50 and 25% of crop evapotranspiration (ET_c), based on Penman–Monteith (PM) method, were tested for their effect on crop growth, crop yield, and water productivity. Tomato (Lycopersicon esculentum, Troy 489 variety) plants were grown in a poly-net greenhouse. Results were compared with the open cultivation system as a control. Two modes of irrigation application namely continuous and intermittent were used. The distribution uniformity, emitter flow rate and pressure head were used to evaluate the performance of drip irrigation system with emitters of 2, 4, 6, and 8 l/h discharge. The results revealed that the optimum water requirement for the Troy 489 variety of tomato is around 75% of the ET_c. Based on this, the actual irrigation water for tomato crop in tropical greenhouse could be recommended between 4.1 and 5.6 mm day⁻¹ or equivalent to 0.3–0.4 l plant⁻¹ day⁻¹. Statistically, the effect of depth of water application on the crop growth, yield and irrigation water productivity was significant, while the irrigation mode did not show any effect on the crop performance. Drip irrigation at 75% of ET_c provided the maximum crop yields and irrigation water productivity. Based on the observed climatic data inside the greenhouse, the calculated ET_c matched the 75–80% of the ET_c computed with the climatic parameters observed in the open environment. The distribution uniformity dropped from 93.4 to 90.6%. The emitter flow rate was also dropped by about 5–10% over the experimental period. This is due to clogging caused by minerals of fertilizer and algae in the emitters. It was recommended that the cleaning of irrigation equipments (pipe and emitter) should be done at least once during the entire cultivation period.     

Coping with drought in irrigated South India: Farmers’ adjustments in Nagarjuna Sagar

Continuous upstream water development in the South Indian Krishna Basin has resulted in declining water availability downstream. Upstream water use is not adjusted to reflect rainfall fluctuations, and downstream farmers of the Nagarjuna Sagar irrigation project in the state of Andhra Pradesh are increasingly vulnerable to water supply shocks. Understanding the adaptive capacity of irrigated command areas to fluctuating water conditions is critical. This paper documents the wide range of adjustments adopted by managers and farmers in Nagarjuna Sagar during a period of fluctuating water availability (2000-2007). Primary and secondary data indicate managerial adjustments such as rotational and timely water supplies to meet critical water demands of standing crops. Farmers responded to changing conditions through: (a) crop diversification, (b) shifting calendars, (c) conjunctive use, (d) suspending cultivation, (e) sale of livestock, (f) out-migration, and (g) tampering with the irrigation system. Adaptive strategies are more diverse in the tail-end than in the head-end of the canal network and local adjustments are often uncoordinated and may degrade the resource base. A better understanding of the practices induced by changes in water availability is needed to refine current water allocation and management in large surface irrigation projects. Crop diversification, deficit irrigation in low-flow years, and conjunctive use are some of the practices to be promoted in a conducive agricultural environment.     

南京市高淳区城市水网水环境改善模拟研究

现代城市多面临水质型缺水问题,城市水系多因污染物超标排放、水系不流通导致水质较差,无法达到相应水功能区水质目标。城市水网的构建是解决区域水资源调配不均、水环境不达标的重要手段之一。以南京市高淳区固城湖、水碧桥河、石固河、官溪河为研究对象,通过构建一维、二维水动力水质耦合模型,选取COD、NH3-N作为水质指标,模拟不同水资源利用情况下研究区域内水体水质分布情况,评估河湖水网连接对水体水质的改善效果。研究表明,河湖水网健康连通及科学调度可显著改善水体水质。