Soil evaporation from drip-irrigated olive orchards

Evaporation from the soil (E_s) in the areas wetted by emitters under drip irrigation was characterised in the semi-arid, Mediterranean climate of Córdoba (Spain). A sharp discontinuity in E_s was observed at the boundary of the wet zone, with values decreasing sharply in the surrounding dry area. A single mean value of evaporation from the wet zone (E_sw) was determined using microlysimeters. Evaporation from the wet zones of two drip-irrigated olive orchards was clearly higher than the corresponding values of E_s calculated assuming complete and uniform soil wetting (E_so), demonstrating the occurrence of micro-scale advection in olive orchards under drip irrigation. Measurements over several days showed that the increase in evaporation due to microadvection was roughly constant regardless of location and of the fraction of incident radiation reaching the soil. Thus, daily evaporation from wet drip-irrigated soil areas (E_sw) could be estimated as the sum of E_so and an additive microadvective term (T_MA). To quantify the microadvective effects, we developed variable local advective conditions by locating a single emitter in the centre of a 1.5 ha bare plot which was subjected to drying cycles. E_sw increased relative to E_so as the soil dried and advective heat transfer increased evaporation from the area wetted by the emitter. The microadvective effects on E_s were quantified using a microadvective coefficient (K_sw), defined as the ratio between E_sw and E_so. A model was then developed to calculate T_MA for different environmental and orchard conditions. The model was validated by comparing measured E_sw against simulated evaporation (E_so+T_MA) for different soil positions and environmental conditions in two drip-irrigated olive orchards. The mean absolute error of the prediction was 0.53 mm day^-1, which represents about a 7% error in evaporation. The model was used to evaluate the relative importance of seasonal E_s losses during an irrigation season under Córdoba conditions. Evaporation from the emitter zones (E_sw) represented a fraction of seasonal orchard evapotranspiration (ET), which ranged from 4% to 12% for a mature (36% ground cover) and from 18% to 43% of ET for a young orchard (5% ground cover), depending on the fraction of soil surface wetted by the emitters. Estimated potential water savings by shifting from surface to subsurface drip ranged from 18 to 58 mm in a mature orchard and from 28 to 93 mm in a young orchard, assuming daily drip applications and absence of rainfall during the irrigation season.     

Irrigation rate and plant density effects on yield and water use efficiency of drip-irrigated corn

The efficient use of water by modern irrigation systems is becoming increasingly important in arid and semi-arid regions with limited water resources. This study was conducted for 2 years (2005 and 2006) to establish optimal irrigation rates and plant population densities for corn (Zea mays L.) in sandy soils using drip irrigation system. The study aimed at achieving high yield and efficient irrigation water use (IWUE) simultaneously. A field experiment was conducted using a randomized complete block split plot design with three drip irrigation rates (I₁: 1.00, I₂: 0.80, and I₃: 0.60 of the estimated evapotranspiration), and three plant population densities (D₁: 48,000, D₂: 71,000 and D₃: 95,000 plants ha⁻¹) as the main plot and split plot, respectively. Irrigation water applied at I₁, I₂ and I₃ were 5955, 4762 and 3572 m³ ha⁻¹, respectively. A 3-day irrigation interval and three-way cross 310 hybrid corn were used. Results indicated that corn yield, yield components, and IWUE increased with increasing irrigation rates and decreasing plant population densities. Significant interaction effects between irrigation rate and plant population density were detected in both seasons for yield, selected yield components, and IWUE. The highest grain yield, yield components, and IWUE were found for I₁D₁, I₁D₂, or I₂D_1, while the lowest were found for I₃D₂ or I₃D₃. Thus, a high irrigation rate with low or medium plant population densities or a medium irrigation rate with a low plant population density are recommended for drip-irrigated corn in sandy soil. Crop production functions with respect to irrigation rates, determined for grain yield and different yield components, enable the results from this study to be extrapolated to similar agro-climatic conditions.     

Water use by an irrigated almond orchard

The evapotranspiration rate of a high-yielding (4.3 t/ha) almond orchard was measured by the eddy covariance technique. The site was subject to advection (LE/Rn > 1) for one-third of the mid-season. The slope of energy balance equation calculated from half-hourly flux data was 0.87. Flux data were transformed by forcing closure of the energy balance to give a seasonal ET of 1,450 mm (ETo 1,257 mm). This value could be reconciled with ancillary measures of soil salinity and water content, and plant water status. The mid-phase crop coefficient was 1.1 which was 0.1 higher than a recently published value. Use of the transformed value of ET in calculations of field application efficiency and annual drainage gives values of 98% and 24 mm, respectively.     

Crop coefficient for drip-irrigated cotton in a Mediterranean environment

A 3-year study was conducted in the eastern Mediterranean region of northern Syria to develop crop coefficient, K _c, for drip-irrigated short-season cotton (Gossypium hirsutum L.). Two sets of K _c curves were determined, the generalized K _c published by the UN’s Food and Agriculture Organization (FAO) that was adjusted for local climate, and the locally developed K _c as the ratio of measured cotton evapotranspiration to calculated reference evapotranspiration. The adjusted FAO K _c curves were the same for the 3 years. However, the locally developed K _c curves not only differed among the 3 years, but also from the adjusted FAO K _c. During the mid-season stage, the adjusted FAO K _c was 24% higher than the locally developed value of 1.05. Variations in locally developed K _c values were caused by normal year-to-year variations in irrigation timing and amount, suggesting sensitivity of K _c that cautions against the use of locally developed K _c based on limited data (i.e., a single season). On the season, the overestimation of crop evapotranspiration by using adjusted FAO K _c was substantial and equivalent to 150 mm water or about two additional irrigations per season. Results caution against blind application of published FAO K _c curve, suggesting some local or regional calibration for increased accuracy.     

Effect of full and limited irrigation amount and frequency on subsurface drip-irrigated maize evapotranspiration,yield, water use efficiency and yield response factors

The objectives of this study were to: (1) to evaluate the effects of subsurface drip irrigation amount and frequency on maize production and water use efficiency, (2) develop production functions and quantify water use efficiency, and (3) develop and analyze crop yield response factors (Ky) for field maize (Zea mays L.). Five irrigation treatments were imposed: fully irrigated treatment (FIT), 25 % FIT, 50 % FIT, 75 % FIT, rainfed and an over-irrigation treatment (125 % FIT). There was no significant (P > 0.05) difference between irrigation frequencies regarding the maximum grain yield; however, at lower deficit irrigation regime, medium irrigation frequency resulted in lower grain yield. There was a decrease in grain yield with the 125 % FIT as compared to the FIT, which had statistically similar yield as 75 % FIT. Irrigation rate significantly impacted grain yield in 2005, 2006 and 2007, while irrigation frequency was only significant during the 2005 and 2006 growing seasons (two dry years) and the interacting effect was only significant in the driest year of 2005 (P = 0.006). For the pooled data from 2005 to 2008, irrigation rate was significant (P = 0.001) and irrigation frequency was also significant (P = 0.015), but their interaction was not significant (P = 0.207). Overall, there were no significant differences between irrigation frequencies in terms of grain yield. Ky had interannual variation and average seasonal Ky values were 1.65, 0.91, 0.91 and 0.83 in 2005, 2006, 2007 and 2008, respectively, and the pooled data (2005–2008) Ky value were 1.14.     

Water use by alfalfa,maize, and barley as influenced by available soil water

The availability of soil water is one of the most important determinants of crop production. Field studies were conducted to examine the relationships between relative evapotranspiration ($\text{E}\text{E}{}_{\mathrm{<mtext>max</mtext>}}$) and available water (W) for alfalfa, maize, and barley. Line source sprinkler irrigation systems were used to provide the variations in soil moisture. Actual evapotranspiration (E) was determined using the water balance method. Maximum evapotranspiration (E_max) was the highest E observed among all irrigation levels. Potential evapotranspiration (E₀) was estimated using Penman's equation to characterize the evaporative demand.The results show that the relationships between $\text{E}\text{E}{}_{\mathrm{<mtext>max</mtext>}}$ and W were different for the three crops. For alfalfa, the relationship was dependent on the physical properties of the soil and on E₀. In a clay loam soil, the decline in E from E_max commenced at a higher value of W than in a sandy loam soil. Furthermore, the rate of decline in E from E_max was dependent on E₀ and was greater as E₀ increased. In the sandy loam soil, the relationship between $\text{E}\text{E}{}_{\mathrm{<mtext>max</mtext>}}$ and W was not dependent on E₀. For maize and barley in clay loam soils, $\text{E}\text{E}{}_{\mathrm{<mtext>max</mtext>}}$ as a function of W was linear, and was not dependent on E₀. This study was compared to results reported in the literature, and it was hypothesized that differences were related mainly to the way variation in soil moisture was introduced over the measurement period.     

Modeling soil water dynamics in a drip-irrigated intercropping field under plastic mulch

Intercropping, drip irrigation, and the use of plastic mulch are important management practices, which can, when utilized simultaneously, increase crop production and save irrigation water. Investigating soil water dynamics in the root zone of the intercropping field under such conditions is essential in order to understand the combined effects of these practices and to promote their wider use. However, not much work has been done to investigate soil water dynamics in the root zone of drip-irrigated, strip intercropping fields under plastic mulch. Three field experiments with different irrigation treatments (high T1, moderate T2, and low T3) were conducted to evaluate soil water contents (SWC) at different locations, for different irrigation treatments, and with respect to dripper lines and plants (corn and tomatoes). Experimental data were then used to calibrate the HYDRUS (2D/3D) model. Comparison between experimental data and model simulations showed that HYDRUS (2D/3D) described different irrigation events and SWC in the root zone well, with average relative errors of 10.8, 9.5, and 11.6 % for irrigation treatments T1, T2, and T3, respectively, and with corresponding root mean square errors of 0.043, 0.035, and 0.040 cm³ cm^?3, respectively. The results showed that the SWC in the shallow root zone (0–40 cm) was lower under non-mulched locations than under mulched locations, irrespective of the irrigation treatment, while no significant differences in the SWC were observed in the deeper root zone (40–100 cm). The SWC in the shallow root zone was significantly higher for the high irrigation treatment (T1) than for the low irrigation treatment, while, again, no differences were observed in the deeper root zone. Simulations of two-dimensional SWC distributions revealed that the low irrigation treatment (T3) produced serious severe water stress (with SWCs near the wilting point) in the 30–40 cm part of the root zone, and that using separate drip emitter lines for each crop is well suited for producing the optimal soil water distribution pattern in the root zone of the intercropping field. The results of this study can be very useful in designing an optimal irrigation plan for intercropped fields.     

Crop coefficients and actual evapotranspiration of a drip-irrigated Merlot vineyard using multispectral satellite images

A field experiment was carried out to evaluate the METRIC (mapping evapotranspiration at high resolution with internalized calibration) model to estimate the actual evapotranspiration (ET_a) and crop coefficient (K _c) of a drip-irrigated Merlot vineyard during the 2007/2008 and 2008/2009 growing seasons. The Merlot vineyard located in the Talca Valley (Chile) was trained on a vertical shoot positioned system. The performance of METRIC was evaluated using measurements of ET_a and K _c from an eddy covariance (EC) system. METRIC overestimated ET_a by about 9?% with a root mean square error (RMSE) and mean absolute error (MAE) of 0.62 and 0.50?mm?d^?1, respectively. For the main phenological stages of the Merlot vineyard, METRIC overestimated the K _c by about 10?% with RMSE?=?0.10 and MAE?=?0.08. Furthermore, the indexes of agreement were 0.70 for K _c and 0.85 for ET_a. Mean values of K _c measured from EC were 0.41, 0.53, 0.56, and 0.46, while those estimated by METRIC were 0.46, 0.54, 0.59, and 0.62 for the bud break to flowering, flowering to fruit set, fruit set to veraison, and veraison to harvest stages, respectively.     

The effect of drip line placement on yield and quality of drip-irrigated processing tomatoes

Subsurface drip irrigation of processing tomatoes is increasing in California. The common design approach is to bury drip lines 0.2–0.36 m deep in the middle of the plant row, which places drip lines directly beneath plant rows. This design limits the use of the drip irrigation system to only those crops compatible with this drip line and bed spacing, and thus, other design approaches are being investigated to increase the flexibility of the drip systems. These approaches are installing drip lines in alternate furrows and installing drip lines in every furrow, both of which place drip lines midway between plant rows. The furrows are the result of the cultural practices used to form beds for planting.This study investigated the effect of the different drip line placements on crop yield and quality. Results showed that the highest yields occurred for the buried placement and the smallest yields for the alternate furrow placement. For the buried placement, soil water content and root density were concentrated around the drip lines, directly beneath the plant rows, while for the furrow placements, zones of high soil water content and root density did not coincide with the plant rows. However, some growers have found the furrow placement to reduce some of the disease problems normally experienced with the traditional furrow irrigation methods.     

Estimation of actual evapotranspiration for a drip-irrigated Merlot vineyard using a three-source model

A study was performed in order to evaluate the three-source model (Clumped model) for direct estimation of actual evapotranspiration (ET_a) and latent heat flux (LE) over a drip-irrigated Merlot vineyard trained on a vertical shoot positioned system (VSP) under semi-arid conditions. The vineyard, with an average fractional cover of 30%, is located in the Talca Valley, Region del Maule, Chile. The performance of the Clumped model was evaluated using an eddy covariance system during the 2006/2007 and 2007/2008 growing seasons. Results indicate that the Clumped model was able to predict ET_a with a root mean square error (RMSE), mean bias error (MBE), and model efficiency (EF) of 0.33, −0.15 mm day⁻¹ and 74%, respectively. Also, the Clumped model simulated the daytime variation of LE with a RMSE of 36 W m⁻², MBE of −8 W m⁻², and EF of 83%. Major disagreement (underestimated values) between observed and estimated values of ET_a was found for clear days after rainfall or foggy days, but underestimated values were less than 10% of the data analysis. The results obtained in this study indicate that the Clumped model could be used to directly estimate vine water requirements for a drip-irrigated vineyard trained on a VSP. However, application of the Clumped model requires a good characterization of the drip-irrigated vineyard architecture.     

Optimal coupling combinations between irrigation frequency and rate for drip-irrigated maize grown on sandy soil

This study was conducted over 2 years (2007 and 2008) to establish the optimal combinations between irrigation frequency and rate for drip-irrigated maize using water production functions and water use-yield relationships. A field experiment was conducted using a randomized complete block split plot design with four irrigation frequencies (F₁, F₂, F₃ and F₄, irrigation events once every 1, 2, 3 or 4 days, respectively) and three drip irrigation rates (I₁: 1.00, I₂: 0.80, and I₃: 0.60 of the estimated evapotranspiration, ET) as the main and split plots, respectively. Our results show that yield variables and water use efficiencies (WUEs) increased with increasing irrigation frequency and rate, with non-significant differences between F₁ and F₂ in yield variables and between I₁ and I₂ in WUEs. Moreover, the combination between various irrigation frequencies and rates had an important effect on yield variables and WUEs, with the highest values being found for F₁I₂ and F₂I₁ and the lowest for F₃I₃ and F₄I₃. The F₁I₃ treatment had grain yield and yield components values similar to those obtained for the F₃I₂ and F₄I₁ treatments and WUEs values similar to those obtained for the F₂I₁ and F₂I₂ treatments. Seasonal yield response factors (k_y) were 1.81 and 1.86 in 2007 and 2008, respectively. Production functions of yield versus seasonal crop ET were linear for all combinations of irrigation frequency and rate and for all irrigation frequency treatments with the exception of the F₁ treatment, which instead showed a second order relationship. The relationship between WUE and grain yield was best represented by a power equation. In conclusion, we identified the optimal coupling combinations between irrigation frequency and water application rate to achieve the maximum yield and WUEs under either sufficient (F₂I₁) or limited irrigation (F₁I₃) water supplies.     

Water use of mature Thompson Seedless grapevines in California

Water use of Thompson Seedless grapevines was measured with a large weighing lysimeter from 4 to 7 years after planting (1990-1993). Above-ground drip-irrigation was used to water the vines. Vines growing within the lysimeter were pruned to four and six fruiting canes for the 1990 and 1991 growing seasons, respectively, and eight fruiting canes in the last 2 years. Maximum leaf area per vine at mid-season ranged from 23 to 27 m² across all years. Reference crop evapotranspiration (ET_o) averaged 1,173 mm between budbreak and the end of October each year, with a maximum daily amount of approximately 7 mm each year. Maximum daily vine water use (ET_c) was 6.1, 6.4, 6.0, and 6.7 mm (based upon a land area per vine of 7.55 m²) for 1990, 1991, 1992, and 1993, respectively. Seasonal ET_c was 718 mm in 1990 and ranged from 811 to 865 mm for the remaining 3 years of the study. The differences in water use among years were probably due to the development of the vine's canopy (leaf area), since they were pruned to differing numbers of fruiting canes. These differences were more pronounced early in the season. Soil water content (SWC) within the lysimeter decreased early in the growing season, prior to the initiation of the first irrigation. Once irrigations commenced, SWC increased and then leveled off for the remainder of the season. The maximum crop coefficient (K_c) calculated during the first year (1990) was 0.87. The maximum K_c in 1991, 1992, and 1993 was 1.08, 0.98, and1.08, respectively. The maximum K_c in 1991 and 1993 occurred during the month of September, while that in 1992 was recorded during the month of July. The seasonal K_c followed a pattern similar to that of grapevine leaf area development each year. The K_c was also a linear function of leaf area per vine using data from all four growing seasons. The decrease in K_c late in the 1991, 1992, and 1993 growing seasons, generally starting in September, varied considerably among the years. This may have been associated with the fact that leafhoppers (Erythroneura elegantula Osborn and E. variabilis Beamer) were not chemically controlled in the vineyard beginning in 1991.     

Water use — Yield relations for cowpea and maize

Three cowpea varieties and one maize variety were subjected to varying irrigation treatments, ranging from water deficits to over-irrigation, on a silty loam soil classified as an Alfisol at Ile-Ife, Nigeria. There was a strong curvilinear relation between cowpea yield and evapotranspiration (R² = 0.86 for dry matter yield and R² = 0.87 for dry seed yield). The values of the correlation coefficient dropped to 0.62 and 0.66 for dry matter and seed yields, respectively, when a linear relation was used. When data for over-irrigated fields were omitted from the calculation, a linear relationship yielded R² values close to unity (R² = 0.99). Similar results were obtained on maize dry matter and grain yields in relation to evaporation.     

Water use of spring wheat to raise water productivity

In semi-arid environments with a shortage of water resources and a risk of overexplotation of water supplies, spring wheat (Triticum aestivum L.) is a crop that can reduce water use and increase water productivity, because it takes advantage of spring rainfall and is harvested before the evaporative demands of summer. We carried out an experiment in 2003 at “Las Tiesas” farm, located between Barrax and Albacete (Central Spain), to improve accuracy in the estimation of wheat evapotranspiration (ETc) by using a weighing lysimeter. The measured seasonal ETc averages (5.63 mm day⁻¹) measured in the lysimeter was 417 mm compared to the calculated ETc values (5.31 mm day⁻¹) calculated with the standard FAO methodology of 393 mm. The evapotranspiration crop coefficient (Kc) derived from lysimetric measurements was Kc-mid: 1.20 and Kc-end: 0.15. The daily lysimeter Kc values were fit to the evolution linearly related to the green cover fraction (fc), which follows the crop development pattern. Seasonal soil evaporation was estimated as 135 mm and the basal crop coefficient approach was calculated in this study, Kcb which separates crop transpiration from soil evaporation (evaporation coefficient, Ke) was calculated and related to the green cover fraction (fc) and the Normalized Difference Vegetation Index (NDVI) obtained by field radiometry in case of wheat. The results obtained by this research will permit the reduction of water use and improvement of water productivity for wheat, which is of vital importance in areas of limited water resources.     

Water use and technical efficiencies in horticultural greenhouses in Tunisia

We measure the technical efficiency of unheated greenhouse farms in Tunisia, and propose a measure for irrigation water use efficiency (IWUE) using an alternative form of the data envelopment analysis (DEA) model. Technical efficiency measures the degree to which (all) farm inputs are used efficiently. IWUE is a measure of the efficiency of irrigation water use when other inputs and output are kept constant. As a second stage, a tobit model is used to identify the degree to which technical efficiency and IWUE correlate with a set of explanatory variables. A comparison of the efficiency scores obtained from constant returns to scale (CRS) and variable returns to scale (VRS) specifications shows that most farmers in our sample are producing at an efficient scale. Under the CRS assumption, the average technical efficiency of the sample was 67.3%. A similar pattern of scores was shown for IWUE; although in this case the average IWUE was even lower (42%). This implies that when all other inputs remain constant, the current output could be produced using, on average, 58% less irrigation water. We conclude that farmers’ technical training in greenhouse management, investments in water saving technologies and the existence of a fertigation technique on farm have a significant and positive effect on their level of IWUE. However, IWUE is significantly and negatively affected by the proportion of total farm land allocated to greenhouses.     

Water use and water use efficiency of sweet corn under different weather conditions and soil moisture regimes

Plant growth and development are influenced by weather conditions that also affect water use (WU) and water use efficiency (WUE) and ultimately, yield. The overall goal of this study was to determine the impact of weather and soil moisture conditions on WU and WUE of sweet corn (Zea mays L. var rugosa). An experiment consisting on three planting dates was conducted in 2006 at The University of Georgia, USA. A sweet corn genotype sh2 was planted on March 27 under irrigated and rainfed conditions and on April 10 and 25 under irrigated conditions only. Soil moisture was monitored using PR2 probes. Rainfall and irrigation were recorded with rain gauges installed in the experimental area while other weather variables were recorded with an automatic weather station located nearby. A water balance was used to obtain the crop's daily evapotranspiration (ETc). WUE was calculated as the ratio of fresh and dry matter ear yield and cumulative ETc. The potential soil moisture deficit (Dp) approach was used to determine the crop's moisture stress. Results were analyzed using a single degree freedom contrast, linear regression, and the least significant difference. WU and WUE of sweet corn were both markedly affected by the intra-seasonal weather variability and Dp. For both variables, significant (p < 0.05) differences were found between planting dates under irrigated conditions and between the irrigated and rainfed treatments. WU was as high as 268 mm for the April 10 planting date under irrigated conditions and as low as 122 mm for the March 27 planting date under rainfed conditions. The maximum soil moisture deficit was reached at the milky kernel stage and was as high as 343 mm for the March 27 planting date under rainfed conditions and as low as 260 mm for the April 10 planting date under irrigated conditions. Further work should focus on the impact of the intra-seasonal weather variability and soil moisture conditions during different crop stages to determine critical periods that affect yield.     

Determination of optimum irrigation water amount for drip-irrigated muskmelon (Cucumis melo L.) in plastic greenhouse

This study was conducted to determine the optimum irrigation water amounts for muskmelon (Cucumis melo L.) in plastic greenhouse. The irrigation water amounts were determined based on the percentage of field water capacity. On the same basis of irrigation start-point of 60% (the percent to comparing to the field water capacity), there were four different irrigation water levels 100% (T100), 90% (T90), 80% (T80) and 70% (T70) as the four different treatments. The results showed that plant growth, fruit production and quality were significantly affected under different irrigation water amounts. Plant height and stem diameter decreased as well as fruit yield from treatment T100 to T70. Fruit quality was the best in the T90 treatment. The irrigation water use efficiency (IWUE) values found in this experiment showed that the lower the amount of irrigation water applied, the higher the irrigation water use efficiency obtained.Hence, based on the quality and quantity of muskmelon yield, the regime for 90% of field water capacity is the suitable soil irrigation treatment (T90) which can save irrigation water and improve the quality of fruit. Combined the crop yield, quality and pan evaporation inside greenhouse, obtained Kcp = 1.00 values can be recommended for the most appropriate irrigation scheduling, irrigation water amount is better between T100 and T90. Therefore, applying water by drip irrigation in relation to the amount of water evaporated from a standard 0.2 m diameter pan is a convenient, simple, easy, and low cost method inside a plastic greenhouse.     

地下滴灌不同土壤水分调控对马铃薯产量和灌溉水利用效率的影响

为了探索地下滴灌作物适宜的土壤水分调控方法,以马铃薯为研究对象,大田试验地下滴灌在滴灌带埋深为30cm的条件下,控制地下20cm处4个不同土壤基质势下限(-10、-30、-40、-50kPa)对马铃薯产量、块茎质量分级占比和灌溉水利用效率等的影响。结果表明,马铃薯的株高随着土壤基质势的降低而降低;土壤基质势分别控制在-10、-30和-40kPa以上时,马铃薯产量在处理间不存在显著差异,当土壤基质势降低到-50kPa之下时,马铃薯产量较-40kPa处理降低46.8%;-50kPa处理的灌溉水利用效率最高,其次是-30和-40kPa处理;-30kPa处理的马铃薯块茎高质量占比大幅高于其他处理。因此,当滴灌带埋设于地下30cm时,控制地下20cm深度处的土壤基质势下限为-30kPa时,可以取得最佳效果。该研究可为指导地下滴灌灌溉计划制订方法提供参考。     

静水法渠道测渗计算

通过静水法测渗研究了渠道渗漏特性，提出了相应的渠道测渗计算方法。实测资料分析表明，此法具有一定的理论基础，可以消除测量误差以提高成果精度，能够计算任一运用水位下的渗漏强度，优于分段计算法。     

Water use in rice schemes in the Senegal river Delta and Valley

The water use of rice schemes was derived from field measurements made during the 1991 and 1992 season at 13 irrigation schemes situated in the delta and valley of the Senegal river. The seasonal water consumption, as well as the water distribution throughout the season and the operation of the pumping stations were evaluated. The water consumption was compared with standards which ranged from 16,650 to 21,100 m³ ha^–1 depending on the season and the location of the scheme. Half of the analyzed schemes used more water than required. Analysis of the water distribution throughout the season revealed that in general the supply follows the demand. In small-scale schemes however, the fit between supply and demand is often lacking, resulting in crop stress and yield reduction. Furthermore, the study shows that the capacity of half of the pumping stations are under-used as a result of a long stretched saturation period and/or the irrigation of only a small fraction of the total area of the scheme.