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.     

Combining hydrological modeling and GIS approaches to determine the spatial distribution of groundwater recharge in an arid irrigation scheme

Accurate quantification of the rate of groundwater (GW) recharge, a pre-requisite for the sustainable management of GW resources, needs to capture complex processes, such as the upward flow of water under shallow GW conditions, which are often disregarded when estimating recharge at a larger scale. This paper provides (1) a method to determine GW recharge at the field level, (2) a consequent procedure for up-scaling these findings from field to irrigation scheme level and (3) an assessment of the impacts of improved irrigation efficiency on the rate of GW recharge. The study is based on field data from the 2007 growing season in a Water Users Association (WUA Shomakhulum) in Khorezm district of Uzbekistan, Central Asia, an arid region that is characterized by a predominance of cotton, wheat and rice under irrigation. Previous qualitative studies in the region reported irrigation water supplies far above the crop water requirements, which cause GW recharge. A field water balance model was adapted to the local irrigation scheme; recharge was considered to be a fraction of the irrigation water losses, determined as the difference between net and gross irrigation requirements. Capillary rise contribution from shallow GW levels was determined with the HYDRUS-1D model. Six hydrological response units (HRUs) were created based on GW levels and soil texture using GIS and remote sensing techniques. Recharge calculated at the field level was up-scaled first to these HRUs and then to the whole WUA. To quantify the impact of improved irrigation efficiency on recharge rates, four improved irrigation efficiency scenarios were developed. The area under cotton had the second highest recharge (895 mm) in the peak irrigation period, after rice with 2,514 mm. But with a low area share of rice in the WUA of <1 %, rice impacted the total recharge only marginally. Due to the higher recharge rates of cotton, which is grown on about 40 % of the cropped area, HRUs with a higher share of cotton showed higher recharge (9.6 mm day^?1 during August) than those with a lower share of cotton (4.4 mm day^?1). The high recharge rates in the cotton fields were caused by its water requirements and the special treatment given to this crop by water management planners due to its strategic importance in the country. The scenario simulations showed that seasonal recharge under improved irrigation efficiency could potentially be reduced from 4 mm day^?1 (business-as-usual scenario) to 1.4 mm day^?1 (scenario with maximum achievable efficiency). The combination of field-level modeling/monitoring and GIS approaches improved recharge estimates because spatial variability was accounted for, which can assist water managers to assess the impact of improved irrigation efficiencies on groundwater recharge. This impact assessment enables managers to identify options for a recharge policy, which is an important component of integrated management of surface and groundwater resources.     

Modeling the soil water balance based on time-dependent hydraulic conductivity under different tillage practices

A simulation model with time-dependent hydraulic conductivity parameters was used to predict the effects of three different tillage practices: conventional tillage (CT), no-tillage (NT) and subsoiling tillage (ST) on the components of the soil water balance during the summer maize growing season. The predictive capability of the model was improved, particularly for the subsoiling tillage case. The simulation results show that temporal changes in soil hydraulic conductivity induced by different tillage practices can affect percolation, water storage, transpiration and evaporation. Differences in the simulated components of the water balance were found to be small between CT and NT practices, but larger in the ST case. Compared with the conventional and no-tillage methods, subsoiling promotes infiltration and deep percolation, thereby favoring a possible recharge of the groundwater. Actual evaporation is always lower in the subsoiled plots, whatever the hydrological year. Transpiration is similar for the three treatments, suggesting no significant differences in water availability, except in wet years where it is higher in subsoiled soils.     

Importance of water consumption by perennial vegetation in irrigated areas of the humid tropics: evidence from Sri Lanka

In tropical, monsoon climates of South-East Asia, irrigation facilities supplement rain in the wet season and enable crops to be cultivated during the dry season. In the Dry Zone of Sri Lanka, 70% of the average annual rainfall of 1000 mm falls in a 3 month period. During the dry season, reference evapotranspiration has less rainfall — about 700 mm, indicating that much additional supply is meant to support crops, mainly paddy. In this climatic context, irrigation has dramatically changed the local environment, creating ecosystems quite similar to that of the wet zone to flourish. In these systems, recharge of shallow groundwater by percolation from irrigated fields, canals, and tanks, has provided a continuous supply of water for natural vegetation and homestead gardens. Much of the water used by this non-crop vegetation is beneficial. Growth of fruit and coconut trees can be quite profitable, while other trees enhance the environment.In 1998, IWMI performed a comprehensive water balance in the command area of the Kirindi Oya irrigation scheme, Sri Lanka, based on surface flow measurements, rainfall data, and estimation of crop water requirements. This water balance showed that evaporation consumed 78% of the total amount of water available for use. The amount of evaporation is split into process depletion (crops for 28%), direct evaporation from tanks (7%), inter-seasonal fallow (10%) and from non-crop vegetation for 55%.The main conclusion from this study is that perennial vegetation as the main component of non-crop vegetation, is a significant consideration in tropical humid environments in planning, management and performance assessment. Designers, managers, and researchers need to specifically incorporate the evaluation of evaporation by non-crop vegetation and perennial vegetation in their approach of water requirements. Further investigation is needed to estimate water consumption by land cover type to assess their respective beneficial use.     

Irrigation management for groundwater quality protection

Deep percolation flow below agricultural and can transport nitrate and pesticide residues to underlying groundwater. Irrigated agriculture in dry climates can also contaminate groundwater with salt from irrigation water and with trace elements such as selenium leached from the vadose zone. Groundwater contamination by agricultural chemicals can be minimized by using best management practices (BMPs) for crop production (including low-input sustainable agriculture or other source control) and for irrigation. Irrigation systems should be designed and managed for zero or minimum deep percolation during the growing seasons to keep fertilizer and pesticides in the root zone as long as possible. At other times, irrigation efficiencies can be lower to produce enough deep percolation water for leaching salts out of the root zone. Because of spatial variability and preferential flow, however, some deep percolation and movement of chemicals may still occur, even if the irrigation efficiency is 100%. BMPs should be developed to minimize such deep percolation flow.     

Irrigation scheduling of cotton in a climate with uncertain rainfall

Summary An investigation was made of the irrigation requirements of cotton grown in a sub-humid environment with significant but highly variable rainfall. In the first year of the study, no additional yield benefits accrued to subsequent irrigations following a pre-emergent irrigation due to above average rainfall (550 mm) throughout the growing season. In the second year a similar rainfall amount (502 mm) fell but significant yield increases to irrigation resulted due to the uneven distribution of the rainfall. The main effect was associated with later rains which influenced the number of bolls set. The maximum amount of water extracted by cotton from a deep grey cracking clay was 178 mm. It was found that 70% of this amount could be depleted before irrigation without loss of yield. Crop evapotranspiration varied from 607 mm with no irrigation after emergence to 775 mm following three irrigations. Irrigation was associated with significant losses from rainfall runoff. Too frequent irrigation creates a risk that soil will be too wet to permit utilisation of natural rainfall. Therefore, the use of soil water information to maximise the interval between irrigation is proposed as a necessary basis for efficient irrigation management.     

A water balance model to estimate groundwater recharge in Rechna doab,Pakistan

The main objective of this study was to develop a procedure to evaluate various recharge components of a groundwater reservoir to estimate the long term average seasonal groundwater recharge in Rechna doab in the Punjab province of Pakistan. A regional lumped water balance model for the Rechna doab was developed and applied to estimate the long term a seasonal recharge to groundwater reservoir. For comparison, recharge was also estimated by a specific yield method from observed groundwater levels. A water balance study was conducted on seasonal basis (6 months) for a period of 31 years (1960–1990). Recharge estimated by the two methods was found to be in good agreement. The average value of net groundwater recharge during Kharif (April–September) season was found to be some 60 mm. No recharge occurred during Rabi (October–March), rather there was a depletion of the groundwater reservoir during the winter months. Long term average annual depletion of a groundwater reservoir was found to be greater than corresponding value of annual recharge. It was concluded that on a regional basis the groundwater reservoir was being depleted resulting in an average groundwater table of Rechna doab about 2.3 m fall over the 1960–1990 period.     

Decision support framework for assessment of non-point-source pollution of groundwater in large irrigation projects

Agriculture is the main non-point polluter of groundwater in irrigated areas as fertilizers and other agrochemicals are the main contaminants in the water that drains out of the root zone to recharge the aquifer. Nitrates from fertilizers, dissolved in percolation losses from rice fields, are the source of pollution considered. The concentration of nitrates in the percolated water depends on the distributed field water and nitrogen balances over the area. Its concentration in the groundwater depends on the total recharge, pollution loading, groundwater flow and solute transport within the aquifer. The development and application of a GIS based decision support framework that integrates field scale models of these processes for assessment of non-point-source pollution of groundwater in canal irrigation project areas is presented. The GIS is used for representing the spatial variations in input data over the area and map the output of the recharge and nitrogen balance models. The latter are used to provide the spatially distributed recharge and pollutant load inputs to the distributed groundwater flow and transport models, respectively. Alternate strategies for water and fertilizer use can be evaluated using this framework to ensure long-term sustainability of productive agriculture in large irrigation projects. The development and application of the framework is illustrated by taking a case study of a large canal irrigation system in India.     

冬小麦田间根层中氮素迁移转化规律研究

在冬小麦生长期田间试验的基础上，建立了土壤──作物系统中水分运动及不同形态氮素迁移转化的数学模型，该模型考虑了有机氮的矿化、铵氮的硝化与挥发、硝态氮的反硝化以及土壤吸附、作物吸收等多种影响因素，利用溶质扩散──对流方程模拟了冬小麦生长期田间水分、铵氮、硝态氮含量及其分布的变化。模拟模型计算结果与田间试验结果比较说明，数学模型能较好地模拟田间的实际情况。模型计算结果表明，在不同灌水定额情况下，６０ｍｍ／次的灌水量就能基本满足作物生长的需要，而且几乎不造成深层渗漏。增大灌水定额，作物吸收水量的增加十分有限，却可能导致大量水的深层渗漏损失，溶解在土壤水中的硝态氮亦随土壤水往深层移动，作物吸收的氮量有所减少，并且随土壤水的下渗，硝态氮的深层渗漏损失显著增加。     

基于地下水均衡的灌区合理渠井用水比例

基于地下水均衡模型,分析了陕西泾惠渠灌区不同频率典型年的地下水均衡状况,结果表明降水入渗补给、渠系渗漏及田间灌溉入渗补给、井灌回归补给是灌区地下水的主要补给源,占总补给量的85.99%~82.89%;而人工开采是灌区地下水的主要排泄途径,农灌地下水开采量、人畜和工业用水开采量占总排泄量的69.7%~72.86%.以2010年为现状基准年,2020年为规划水平年,结合灌区发展规划,设置了4种不同的灌区发展情景模式,运用所建立的地下水均衡模型计算了不同情景模式下的地下水位埋深,其变化范围为0~0.07 m;以地下水位变幅最小为准则,得出了不同频率典型年合理的渠井用水比例范围为1.49~1.53,从而为灌区地下水资源的高效持续利用提供了依据.     

Assessing the groundwater dynamics and impacts of water saving in the Hetao Irrigation District, Yellow River basin

Water resources allocated to the agricultural sector in the Yellow River basin are being reduced due to severe water scarcity and increased demand by the non-agricultural sectors. In large-scale irrigation districts, the application of water-saving practices, e.g., improving the canal system, using water-saving irrigation technology and adjusting cropping patterns, is required for the sustainable agricultural development and the river basin environmental equilibrium. Adopting water-saving practices leads to lowering the groundwater table and to controlling salinity impacts related to excessive irrigation. However, assessing the effects of water-saving practices on the groundwater system requires further investigation. The Jiefangzha Irrigation Scheme of the Hetao Irrigation District is used as a case study for analyzing the temporal and spatial dynamics of the groundwater table. A lumped parameter groundwater balance model has been developed with this purpose and to assess impacts of various water-saving practices. The model was calibrated with monthly datasets relative to the non-frozen periods of 1997-1999 and validated with datasets from 2000 to 2002. Results indicate that canal seepage and deep percolation account for respectively 48% and 44% of the annual groundwater recharge. Groundwater discharge by direct evaporation and plant roots uptake represents 82% of the total annual groundwater discharge. After validation, the model was applied to assess the impacts of various canal and farm irrigation water-saving practices. It was observed that improvements in the canal system (e.g., canal lining, upgrading the hydraulic regulation and control structures, improving delivery schedules) might lower the groundwater table by 0.28-0.48 m, depending upon the level of implementation of these measures. Higher declines of the groundwater table are predicted when water-saving technologies are applied at both the canal and the farm systems. That decline of the water table favours salinity control and reduces capillary rise, thus reducing the groundwater evaporation and uptake by plant roots; that reduction may attain 128 mm. However, predictions may change depending on the way how water-saving measures are applied, which may be different of assumptions made; therefore, there is the need to perform a follow-up of the interventions in order to update predictions. Results indicate the need for appropriate research leading to improved irrigation management when the decline of the groundwater level will reduce groundwater contribution to vegetation growth.     

Modelling the effects of climate variability and water management on crop water productivity and water balance in the North China Plain

In the North China Plain (NCP), while irrigation using groundwater has maintained a high-level crop productivity of the wheat-maize double cropping systems, it has resulted in rapid depletion of groundwater table. For more efficient and sustainable utilization of the limited water resources, improved understanding of how crop productivity and water balance components respond to climate variations and irrigation is essential. This paper investigates such responses using a modelling approach. The farming systems model APSIM (Agricultural Production Systems Simulator) was first calibrated and validated using 3 years of experimental data. The validated model was then applied to simulate crop yield and field water balance of the wheat-maize rotation in the NCP. Simulated dryland crop yield ranged from 0 to 4.5 t ha⁻¹ for wheat and 0 to 5.0 t ha⁻¹ for maize. Increasing irrigation amount led to increased crop yield, but irrigation required to obtain maximum water productivity (WP) was much less than that required to obtain maximum crop yield. To meet crop water demand, a wide range of irrigation water supply would be needed due to the inter-annual climate variations. The range was simulated to be 140-420 mm for wheat, and 0-170 mm for maize. Such levels of irrigation applications could potentially lead to about 1.5 m year⁻¹ decline in groundwater table when other sources of groundwater recharge were not considered. To achieve maximum WP, one, two and three irrigations (i.e., 70, 150 and 200 mm season⁻¹) were recommended for wheat in wet, medium and dry seasons, respectively. For maize, one irrigation and two irrigations (i.e., 60 and 110 mm season⁻¹) were recommended in medium and dry seasons, while no irrigation was needed in wet season.     

Irrigation planning in cotton through simulation modeling

Experiments were undertaken at CCS Haryana Agricultural University Farm, Sirsa (India) to estimate the optimum irrigation schedule for cotton resulting in minimum percolation losses. The sprinkler line source technique was adopted for creating various irrigation regimes at different crop growth stages. The SWASALT (Simulation of Water And SALT) model after calibration and validation provided water balance components. The wa-ter management response indicators (WMRI's) such as transpiration efficiency E_t/(Irr + P), relative transpiration E_t/E_tp, evapotranspiration efficiency ET/(Irr + P), soil moisture storage change ΔW/W_int (deficit/excess) and percolation loss Perc/(Irr. + P) were evaluated using water balance components as estimated by the simulation study. Under limited water supply conditions, the optimum irrigation depth was found to be 57 mm at crop growth stages with pre-sowing and 1^st irrigation of 120 mm and 80 mm respectively for sandy clay loam underlain by sandy loam soil (Type I). The corresponding values of relative transpiration, transpiration efficiency and evapotranspiration efficiency were 0.65, 0.65 and 0.89 respectively. The crop yield varied linearly with increasing irrigation depth which was evident from increase in relative transpiration with increasing depth of water application. However, increased depth of irrigation resulted in less moisture utilisation from soil storage (20% depletion at 40 mm depth and 4.4% moisture built up at 100 mm depth). The extended simulation study for sandy soil underlain by loamy sand (Type II) indicated that two pre-sowing irrigations each 40 mm and subsequent irrigations of 40 mm at an interval of 20 days depending upon rainfall were optimum. This irrigation scenario resulted in zero percolation loss accompanied by 74% relative transpiration and 14 per cent soil moisture depletion. Received: 20 November 1995     

Overland water and salt flows in a set of rice paddies

Cultivation of paddy rice in semiarid areas of the world faces problems related to water scarcity. This paper aims at characterizing water use in a set of paddies located in the central Ebro basin of Spain using experimentation and computer simulation. A commercial field with six interconnected paddies, with a total area of 5.31 ha, was instrumented to measure discharge and water quality at the inflow and at the runoff outlet. The soil was classified as a Typic Calcixerept, and was characterized by a mild salinity (2.5 dS m⁻¹) and an infiltration rate of 5.8 mm day⁻¹. The evolution of flow depth at all paddies was recorded. Data from the 2002 rice-growing season was elaborated using a mass balance approach to estimate the infiltration rate and the evolution of discharge between paddies. Seasonal crop evapotranspiration, estimated with the surface renewal method, was 731 mm (5.1 mm day⁻¹), very similar to that of other summer cereals grown in the area, like corn. The irrigation input was 1874 mm, deep percolation was 830 mm and surface runoff was 372 mm. Irrigation efficiency was estimated as 41%. The quality of surface runoff water was slightly degraded due to evapoconcentration and to the contact with the soil. During the period 2001–2003, the electrical conductivity of surface runoff water was 54% higher than that of irrigation water. However, the runoff water was suitable for irrigation. A mechanistic mass balance model of inter-paddy water flow permitted to conclude that improvements in irrigation efficiency cannot be easily obtained in the experimental conditions. Since deep percolation losses more than double surface runoff losses, a reduction in irrigation discharge would not have much room for efficiency improvement. Simulations also showed that rice irrigation performance was not negatively affected by the fluctuating inflow hydrograph. These hydrographs are typical of turnouts located at the tail end of tertiary irrigation ditches. In fact, these are the sites where rice has been historically cultivated in the study area, since local soils are often saline-sodic and can only grow paddy rice taking advantage of the low salinity of the irrigation water. The low infiltration rate characteristic of these saline-sodic soils (an experimental value of 3.2 mm day⁻¹ was obtained) combined with a reduced irrigation discharge resulted in a simulated irrigation efficiency of 60%. Paddy rice irrigation efficiency can attain reasonable values in the local saline-sodic soils, where the infiltration rate is clearly smaller than the average daily rice evapotranspiration.     

Spatial and temporal distributions of nitrogen and crop yield as affected by nonuniformity of sprinkler fertigation

Deep percolation and nitrate leaching are important considerations in the design of sprinkler systems. Field experiments were therefore conducted to investigate the influence of nonuniformity of sprinkler irrigation on deep percolation and spatial distributions of nitrogen and crop yield during the growing season of winter wheat at an experiment station in Beijing, China. Three experimental plots of a sandy clay loam soil in the 0–40 cm depth interval and a loamy clay soil below 40 cm were irrigated with a sprinkler irrigation system that had a seasonal averaged Christiansen irrigation uniformity coefficient (CU) varying from 72 to 84%. Except for the fertilizer applied before planting, fertilizer was applied with the sprinkler irrigation system. The corresponding seasonal averaged CU for fertigation varied from 71 to 85%. Daily observation of matrix water potentials in the root zone showed that little deep percolation occurred. Consequently, the effect of sprinkler uniformity on deep percolation was minor during the irrigation season for the soil tested. Intensive gravimetric soil core samplings were conducted several times during the irrigation season in a grid of 5 m × 5 m for each plot to determine the spatial and temporal variation of NH₄-N and NO₃-N contents. Soil NH₄-N and NO₃-N exhibited high spatial variability in depth and time during the irrigation season with CU values ranging from 23 to 97% and the coefficient of variation ranging from 0.04 to 1.06. A higher uniformity of sprinkler fertigation produced a more uniform distribution of NH₄-N, but the distribution of NO₃-N was not related to fertigation. Rather it was related to the spatial variability of NO₃-N before fertigation began. At harvest, the distribution of dry matter above ground, nitrogen uptake, and yield were measured and the results indicated that sprinkler fertigation uniformity had insignificant effects on the parameters mentioned above. Field experimental results obtained from this study suggest that sprinkler irrigation if properly managed can be used as an efficient and environment-friendly method of applying water and fertilizers.     

日光温室晚春茬生菜渗灌技术试验研究

采用埋深 1 0 cm的微孔渗灌管对日光温室晚春茬生菜进行了渗灌试验 ,并与沟灌进行了对比。结果表明 ,晚春茬生菜采用渗灌有明显的节水增产效果 ,与沟灌相比可节水 1 9.0 %、增产 1 5 .4 %。通过与栽培措施相结合采用渗灌成功地进行了生菜的定植。渗灌管浅埋灌水可以使表层土壤较快地湿润 ,并达到蔬菜生长所要求的水分 ,同时显著减少灌溉水的深层渗漏 ,提高灌溉水的利用率。温室生菜的田间蒸散量与温室内的蒸发力有直接关系 ,生育期内的日平均田间蒸散量为 2 .0 8mm/d,比沟灌温室内的高。     

On-farm rainwater and crop management for improving productivity of rainfed areas

Scarcity of water is a critical limitation to adoption of modern technology for increasing productivity of traditional rainfed rice growing areas of eastern Madhya Pradesh, India. The shortage of water results from uneven distribution of rains, significant gaps between rain events and field water losses rather than from low seasonal or annual rainfall totals. A feasible strategy to alleviate this limitation is to harvest excess rainwater in a farm pond during the wet season and use the conserved water for crop production in both wet (as insurance against drought) and dry seasons by adopting suitable crop and cropping systems. The results of water balance in a 1.05 ha field, on which a farm pond was built using 0.09 ha area, showed that 28–37% of seasonal rainfall was available as surface runoff from microcatchment (0.66 ha growing soybean, peanut and pigeonpea) for collection in the pond. This was sufficient for saving rice in a 0.30 ha area (in the lower side of the field) from drought stress, and for establishment of chickpea and mustard (in 0.90 ha) in the post-rainy season after harvest of rainy season crops. Soybean, peanut and pigeonpea, grown in the microcatchment during the rainy season, utilized respectively 371–726, 364–733 and 535–920 mm water in evapotranspiration (E,) and deep percolation (P). Rice grown below the pond required 28–317 mm water in different seasons to save the crop from in-season drought stress which commonly occurred during vegetative and reproductive stages. Water requirement (E, + P) of rice was 816–1342 mm in different seasons. Residual soil moisture after rainy season soybean, peanut and rice was sufficient (172–203 mm) to support post rainy season crops of chickpea and mustard. However, the losses of moisture from the soil surface layer after harvest of rainy season crops were rapid (7–23 mm), which necessitated a light irrigation (21–45 mm) for establishment of chickpea and mustard in the post-rainy season. The water balance results of soybean-mustard, peanut-mustard and peanut-chickpea were near identical to soybean-chickpea cropping. Similarly the water balance of rice-mustard was identical to Corresponding author. rice-chickpea in the vertisols. Soybean-mustard and rice-chickpea were the suitable and economical cropping systems for the microcatchment and service area of the farm pond.     

Modeling irrigated cotton with shallow groundwater in the Aral Sea Basin of Uzbekistan: I. Water dynamics

In Khorezm, a region located in the Aral Sea basin of Uzbekistan, water use for irrigation of predominantly cotton is high whereas water use efficiency is low. To quantify the seasonal water and salt balance, water application, crop growth, soil water, and groundwater dynamics were studied on a sandy, sandy loam and loamy cotton field in the years 2003 and 2005. To simulate and quantify improved management strategies and update irrigation standards, the soil water model Hydrus-1D was applied. Results showed that shallow groundwater contributed a substantial share (up to 399 mm) to actual evapotranspiration of cotton (estimated at 488–727 mm), which alleviated water stress in response to suboptimal quantities of water applied for irrigation, but enhanced concurrently secondary soil salinization. Thus, pre-season salt leaching becomes a necessity. Nevertheless, as long as farmers face high uncertainty in irrigation water supply, maintaining shallow groundwater tables can be considered as a safety-net against unreliable water delivery. Simulations showed that in 2003 around 200 mm would have been sufficient during pre-season leaching, whereas up to 300 mm of water was applied in reality amounting to an overuse of almost 33%. Using some of this water during the irrigation season would have alleviated season crop-water stress such as in June 2003. Management strategy analyses revealed that crop water uptake would only marginally benefit from a permanent crop residue layer, often recommended as part of conservation agriculture. Such a mulch layer, however, would substantially reduce soil evaporation, capillary rise of groundwater, and consequently secondary soil salinization. The simulations furthermore demonstrated that not relying on the contribution of shallow groundwater to satisfy crop water demand is possible by implementing timely and soil-specific irrigation scheduling. Water use would then not be higher than the current Uzbek irrigation standards. It is argued that if furrow irrigation is to be continued, pure sandy soils, which constitute <5% of the agricultural soils in Khorezm, are best to be taken out of annual cotton production.     

Modelling of water and nitrogen balances in the ponded water and soil profile of rice fields in Northern Greece

This paper presents a water and nitrogen balance model for the surface ponded water and soil profile system of rice (Oryza sativa L.) fields. The model estimates the daily water balance components, as well as, the daily losses and transformations of nitrogen. Data from two neighbouring rice fields during the growing season of 2005 in the Thessaloniki plain of Northern Greece were used for the application of the model. The data set of field A was used for the calibration of the model, while the data set from the field B for validation of model. Simulation results of total inorganic nitrogen in the soil and runoff water exhibited reasonable agreement with the measured data during calibration and verification of the model. Significant amounts of applied irrigation water were lost through surface runoff and deep percolation into the groundwater. The sum of nitrogen inputs from fertilization, mineralization and irrigation water were 292.7 and 280.4 kg ha⁻¹ for field A and B, respectively. Nitrogen uptake by algae in ponding water and plants was one of the main processes of nitrogen reduction in the rice field systems with an amount of 125.7 and 131.8 kg ha⁻¹ for field A and B, respectively. Leaching through percolated water was the other significant process with 118.3 and 120.8 kg ha⁻¹, respectively. Gaseous losses of nitrogen (via volatilization and denitrification) were also substantial processes of nitrogen reduction in the flooded compartment. The study showed that the simple model presents important results for the water and nitrogen management in rice fields. This information can be used for irrigation water saving and prevention of water resources contamination in rice-based agroecosystems.     

农业节水措施对地下水涵养的作用及其敏感性分析

以北京市大兴区为研究区,利用经校验的水平衡模型,通过调整灌溉满足率和灌溉水利用系数,探讨了不同农业节水措施对增加地下水补给量和减少地下水开采量的作用及其敏感性。结果表明,不同水文年型下,降低灌溉满足率及提高灌溉水利用系数都能减少地下水开采量,且降低灌溉满足率对减少地下净开采量的作用更为显著,有利于区域地下水涵养。在参数取值范围内,地下水净开采量对灌溉满足率的敏感性较大,而地下水补给量对灌溉水利用系数的敏感性较高。与提高灌溉水利用系数相比,对资源性缺水区域,采用先进节水技术,适度降低区域灌溉满足率,对促进水资源持续有效利用及加大地下水涵养具有更显著的效果。