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.     

Potato water use and yield under furrow irrigation

Field experiments were conducted to study the effects of plant-furrow treatments and levels of irrigation on potato (Solanum tuberosum L.) water use, yield, and water-use efficiency. The experiments were carried out under deficit irrigation conditions in a sandy loam soil of eastern India in the winter seasons of 1991/92, 1992/93, and 1993/94. Two plant-furrow treatments and two levels of irrigation were considered. The two plant-furrow treatments were F₁ - furrows with single row of planting in each ridge with 45 cm distance between adjacent ridges, and F₂ - furrows with double rows of planting spaced 30 cm apart in each ridge with 60 cm distance between adjacent ridges. The two levels of irrigation (LOI) were I₁ - 0.9 IW/CPE and I₂ - 1.2 IW/CPE, where IW is irrigation water of 5 cm and CPE is cumulative pan evaporation. Treatment F₂ produced highest tuber yield in all years with average value of 10,610 kg ha ^-1 and 12,780 kg ha ^-1 at LOI of I₁ and I₂, respectively. On average, six irrigations with a total of 25 cm, and seven irrigations with a total of 30 cm were required for both treatments F₁ and F₂ at LOI of I₁ and I₂, respectively. Treatment F₂ resulted in a significantly higher number of branches and tubers per plant, foliage coverage and water-use efficiency for both irrigation levels than treatment F₁. Average daily crop evapotranspiration was found to range from 1.1 to 3.4 mm and from 1.2 to 3.9 mm for treatment F₁ and from 1.1 to 3.6 mm and from 1.2 to 4.0 mm for treatment F₂ at LOI of I₁ and I₂, respectively.     

Comparison of water status indicators for young peach trees

We measured a series of physiological and physical indicators and compared them to xylem sap flow, to identify the most sensitive and reliable plant water status indicator. In the growing season of 1998, 4-year-old peach trees (Prunus persica Batsch cv. 'Suncrest', grafted on 'GF 677' rootstock) were studied under two irrigation treatments, 25 l day^у and no irrigation, and during recovery. Trials were conducted near Pisa (Italy) in a peach orchard situated on a medium clay loam soil and equipped with a drip-irrigation system (four 4 l h^у drippers per tree). Measurements of leaf water potential (ƒ_W), stem water potential (ƒ_S), and leaf temperature (T_l) were taken over 5 days (from dawn to sunset) and analyzed in conjunction with climatic data, sap flow (SF), trunk diameter fluctuation (TDF) and soil water content (SWC). Physiological indicators showed substantial differences in sensitivity. The first indication of changes in water status was the decrease of stem radial growth. TDF and SF revealed significant differences between the two irrigation treatments even in the absence of differences in pre-dawn leaf water potential (pdƒ_W), up until now widely accepted as the benchmark of water status indicators. Irrigated trees showed a typical trend in SF rate during the day, while in non-irrigated plants the maximum peak of transpiration was anticipated. Measurements of water potential showed ƒ_S to be a better indicator of tree water status than ƒ_W. T_l was found to have poor sensitivity. In conclusion, we found the sensitivity of the indicators from the most to the least was: TDF >SF rate >SF cumulated = pdƒ_W=ƒ_S>mdƒ_W>T_l.     

Determination of evapotranspiration for maize and berseem clover

Maize and berseem are among the most important crops in India and several other countries in the world. Irrigation is provided to these crops to get higher production; hence, determining the water requirements of these crops is important for irrigation planning. Improved water management of these crops requires accurate scheduling of irrigation, which in turn requires accurate measurement of crop evapotranspiration (ET_c). Thus, the first objective of this study was to measure daily, weekly and seasonal ET_c of maize and berseem directly from weighing type lysimeters. Experiments were conducted in a set of two electronic weighing-type lysimeters of 7.82 m³ to measure the hourly ET_c of maize and berseem from June 1996 to April 1998 at Karnal, India. The average daily ET_c of maize varied from <2.8 mm day^-1 in the early growing period to >4 mm day^-1 at development and reproductive stages. The peak daily ET_c of maize was 7.7 mm day^-1 and this occurred 9 weeks after sowing (WAS) at the silking stage of maize when leaf area index (LAI) was 5.5. The measured seasonal ET_c of maize was 354 mm. In the case of berseem, the average daily ET_c was 0.9 mm day^-1 at the initial stage, achieved a peak value of 6.9 mm day^-1 between 25 and 26 WAS during the fifth cut. The measured seasonal ET_c of berseem was 480 mm. Precise information on the crop coefficient, which is required for regional-scale irrigation planning, is lacking for semi-arid climates such as those found in north India. Therefore, the second objective of this study was to develop crop coefficients (K_c) for maize and berseem from ET_c measurements and weather data. The estimated values of K_c for maize by the Penman-Monteith method at the four crop growth stages; namely, initial, crop development, mid-season and maturity, were 0.55, 1.00, 1.23 and 0.64, respectively, and the corresponding values for berseem were 0.76, 0.82, 1.11 and 1.24, respectively. In the case of these two crops, actual K_c values determined from this study are different from those suggested by the FAO (Allen et al. 1998), indicating the need for generating these values at the local/ regional level.     

Plant indicators for scheduling irrigation of young olive trees

The sensitivity of several water status indicators was determined in irrigated young olive trees subjected to two drought periods at Cordoba, Spain. Trunk diameter fluctuations (TDF) were monitored continuously and stem water potential (N), leaf photosynthesis (P_n) and conductance (g_l) were measured periodically on trees where irrigation was interrupted or which were fully irrigated. During the first period of water deprivation in late spring, only some of the TDF-derived parameters were able to detect significant differences caused by water deficits, while there were no differences in stem N, P_n and g_l. All water stress indicators responded during the second drought period in midsummer. However, differences in maximum trunk diameter were detected several days before significant stem N differences of about 0.2 MPa were established between treatments. Stem N differences declined further to 0.6 MPa before differences in leaf P_n and g_l became significant. Of all TDF-derived indices, trunk growth rate was the most sensitive to water deficits while treatment differences in maximum daily shrinkage were insignificant in the young trees. It is concluded that continuous monitoring of trunk diameter provides the most sensitive indicator for accurate, automated irrigation scheduling of young olive trees under intensive production.     

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 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.     

Delineating water management zones in a paddy rice field using a Floating Soil Sensing System

Paddy rice fields are kept inundated during most of the growing period. This requirement is challenging to achieve because of the lack of suitable technologies to detect rapidly percolation prone zones within these fields. The objective of this study was to evaluate a methodology to identify water leakage areas to support precision soil–water management at a within-field level. Therefore, a Floating Sensing System (FloSSy) was designed to record the soil apparent electrical conductivity (EC_a) of a paddy field both under dry and inundated conditions using the electromagnetic induction sensor EM38. Comparison of EC_a data sets obtained under inundated and dry conditions showed that the EC_a measurements under inundated condition (EC_a-i) were more strongly related to soil properties due to the absence of variability in soil moisture and the increased stability of the floating sensing platform. Therefore, we proceeded with the EC_a-i measurements and grouped them into two classes using a fuzzy k-means classification method. These classes showed significant differences in water infiltration: lower EC_a values represented a higher infiltration rate and vice versa. This effect was attributed to differences in soil texture, more specifically the sand content, and its effect on water retention. It was concluded that an EC_a-i survey with FloSSy allowed the detection of soil heterogeneity linked to downward water fluxes which has a potential to support precision soil–water management in inundated fields.     

Water requirements of subsurface drip-irrigated faba bean in California

A 3-year study was done in central California to determine the water requirements for growing faba bean (Vicia faba L.) as a winter cover crop using subsurface drip irrigation (SDI). Water was applied at 0, 50, and 100% of the estimated crop evapotranspiration (ET_c) the first 2 years and 50, 100, and 150% ET_c the third year, with drip laterals installed 0.30, 0.45, or 0.60 m deep. Rainfall was above normal the first year (>330 mm) and irrigation had no effect on crop production. Irrigation improved production and water-use efficiency the following years, however. Production was higher when drip laterals were located at 0.30 or 0.45 m than at 0.60 m depth, even though roots tended to be concentrated near the laterals (later in the season) regardless of depth. Overall, well-irrigated faba bean required 231-297 mm of water to produce 3.0-4.4 t ha^у of dry vegetative biomass.     

Water use of young Thompson Seedless grapevines in California

Water use of Thompson Seedless grapevines during the first 3 years of vineyard establishment was measured with a large weighing lysimeter near Fresno, California. Two grapevines were planted in a 2ǸǶ m deep lysimeter in 1987. The row and vine spacings in the 1.4-ha vineyard surrounding the lysimeter were approximately 3.51 and 2.15 m, respectively. Vines in the lysimeter were furrow-irrigated from planting until the first week of September in 1987. They were subsequently irrigated with subsurface drip-irrigation whenever they had used 2 mm of water, based upon the area of the lysimeter (equivalent to 8 liters per vine). The trellis system, installed the second year, consisted of a 2.13 m long stake, driven 0.45 m into the soil with a 0.6 m cross-arm placed at the top of the stake. Crop coefficients (K_c) were calculated using measured water losses from the lysimeter (ET_c) and reference crop evapotranspiration (ET_o) obtained from a CIMIS weather station located 2 km from the vineyard. Water use of the vines in 1987 from planting until September was approximately 300 mm, based on the area allotted per vine in the vineyard surrounding the lysimeter. Daily water use just subsequent to a furrow-irrigation event exceeded ET_o (>6.8 mm day^у). Water use from budbreak until the end of October in 1988 and 1989 was 406 and 584 mm, respectively. The initiation of subsurface drip-irrigation on 23 May 1988 and 29 April 1989 doubled ET_c measured prior to those dates. Estimates of a 'basal' K_c increased from 0.1 to 0.4 in 1987. The seasonal K_c in 1988 increased throughout the season and reached its peak (0.73) in October. The highest K_c value in 1989 occurred in July. It is suggested that the seasonal and year-to-year variation in the K_c was a result of the growth habit of the vines due to training during vineyard establishment. The results provide estimates of ET_c and K_c for use in scheduling irrigations during vineyard establishment in the San Joaquin Valley of California and elsewhere with similar environmental conditions.     

Quantitative evaluation of soil salinity and its spatial distribution using electromagnetic induction method

In the Lower Yellow River Delta, soil salinity is a problem due to the presence of a shallow, saline water table and marine sediments. Spatial information on soil salinity at the field level is increasingly needed, particularly for better soil management and crop allocation in this area. In this paper, a mobile electromagnetic induction (EMI) system including EM38 and EM31 is employed to perform field electromagnetic (EM) survey, and fast determination and quantitative evaluation of the spatial pattern of soil salinity is discussed using the field EM survey data. Optimal operation modes of EM38 and EM31 are determined to establish multiple linear regression models for estimating salinity from apparent soil electrical conductivity (EC_a). Spatial trend and semivariogram are illustrated and spatial distribution of field salinity status is further visualized and quantitatified. The results suggest that EC_a (EM38 and EM31) data is highly correlated with salinity, and that the interpretation precision of soil salinity at various layers can be improved using EM_38h and EM_31h (where h represents the horizontal mode of EM measurement). Both EM_38h and EM_31h exhibit significant geographic trend. Nested spherical models fit the semivariance of EM_38h and EM_31h better than single spherical models. Spatial autocorrelation of EM_31h is stronger than that of EM_38h, and short-range variation is the chief constitute of spatial heterogeneity for both EM_38h and EM_31h. Quantitative classification shows that soil salinity exhibits the trend of accumulation in the root zone. In 0-1.0 m solum, heavy salinized and saline soils are the predominant soil types, accounting for 54% and 41% of total survey area, respectively. The area of light and moderate salinized soils is comparatively small, which accounts for only 0.4% and 4.6%, respectively.     

Evapotranspiration of orange trees in greenhouse lysimeters

Eight-year-old Murcott orange trees (Citrus sinensis (L.) Murcott) grown in greenhouse lysimeters filled with sandy soil were used to investigate seasonal variations in daily and hourly evapotranspiration. The study was conducted in Japan during the summer of 2000 and the winter of 2001. Weighing lysimeters of 1.5 m diameter and 1.6 m depth (three replications) planted with a tree were irrigated when average soil moisture in 0-120 cm of soil depth was depleted to below 70% of the field capacity (FC). Evapotranspiration (ET) showed significant seasonal variations. Average ET rate exceeded 4.4 mm/day in the summer period, and dropped to 0.6 mm/day in the winter months. The average seasonal crop coefficient (K_C) was 0.91 and 0.75 during the summer and winter periods, respectively. Hourly variations in ET exhibited a time difference with season. The time of maximum ET was 0900 hours for winter and 1200 hours for summer. Moreover, some evaporative losses of soil water occurred even during the night in both summer and winter seasons. Soil evaporation (E) was 33% of ET during the winter period, while E was only 11% of ET during summer. Maximum water uptake by the trees was found at a depth of 30-60 cm, and soil water depletion was observed in the 0-120 cm depth of the profile during the summer period. However, during the winter season, water depletion occurred only from 0-30 cm depth of the soil profile.     

Scheduling irrigations by merging water-use and radar backscatter models

Penman-Monteith methods were merged with the water cloud radar model to simulate soil moisture, canopy moisture and radar backscatter for a field of potatoes (Solanum tuberosum) in the United Kingdom. The objective of this paper is to present the irrigation scheduling potential of the merged model. This study simulates water-use: radar backscatter simulation results are presented elsewhere and yield is not calculated by the merged model. Soil moisture deficit switches control the model, which in turn uses predetermined application rates defined by the user. The simulation results show that the most effective irrigation application during the 1999 simulation period would utilise 25 mm of water when a soil moisture deficit of 30 mm is reached. Significantly, this would have required less water application than that actually applied to the field.     

A method for spatial prediction of daily soil water status for precise irrigation scheduling

Available water holding capacity (AWC) and field capacity (FC) maps have been produced using regression models of high resolution apparent electrical conductivity (EC_a) data against AWC (adj. R² = 0.76) and FC (adj. R² = 0.77). A daily time step has been added to field capacity maps to spatially predict soil water status on any day using data obtained from a wireless soil moisture sensing network which transmitted hourly logged data from embedded time domain transmission (TDT) sensors in EC_a-defined management zones. In addition, regular time domain reflectometry (TDR) monitoring of 50 positions in the study area was used to assess spatial variability within each zone and overall temporal stability of soil moisture patterns. Spatial variability of soil moisture within each zone at any one time was significant (coefficient of variation [% CV] of volumetric soil moisture content (θ) = 3-16%), while temporal stability of this pattern was moderate to strong (bivariate correlation, R = 0.52-0.95), suggesting an intrinsic soil and topographic control. Therefore, predictive ability of this method for spatial characterisation of soil water status, at this site, was limited by the ability of the sensor network to account for the spatial variability of the soil moisture pattern within each zone. Significant variability of soil moisture within each EC_a-defined zone is thought to be due to the variable nature of the young alluvial soils at this site, as well as micro-topographic effects on water movement, such as low-lying ponding areas. In summary, this paper develops a method for predicting daily soil water status in EC_a-defined zones; digital information available for uploading to a software-controlled automated variable rate irrigation system with the aim of improved water use efficiency. Accuracy of prediction is determined by the extent to which spatial variability is predicted within as well as between EC_a-defined zones.     

Soil salinity related to physical soil characteristics and irrigation management in four Mediterranean irrigation districts

Irrigated agriculture is threatened by soil salinity in numerous arid and semiarid areas of the Mediterranean basin. The objective of this work was to quantify soil salinity through electromagnetic induction (EMI) techniques and relate it to the physical characteristics and irrigation management of four Mediterranean irrigation districts located in Morocco, Spain, Tunisia and Turkey. The volume and salinity of the main water inputs (irrigation and precipitation) and outputs (crop evapotranspiration and drainage) were measured or estimated in each district. Soil salinity (EC_e) maps were obtained through electromagnetic induction surveys (EC_a readings) and district-specific EC_a-EC_e calibrations. Gravimetric soil water content (WC) and soil saturation percentage (SP) were also measured in the soil calibration samples. The EC_a-EC_e calibration equations were highly significant (P < 0.001) in all districts. EC_a was not significantly correlated (P > 0.1) with WC, and was only significantly correlated (P < 0.1) with soil texture (estimated by SP) in Spain. Hence, EC_a mainly depended upon EC_e, so that the maps developed could be used effectively to assess soil salinity and its spatial variability. The surface-weighted average EC_e values were low to moderate, and ranked the districts in the order: Tunisia (3.4 dS m⁻¹) > Morocco (2.2 dS m⁻¹) > Spain (1.4 dS m⁻¹) > Turkey (0.45 dS m⁻¹). Soil salinity was mainly affected by irrigation water salinity and irrigation efficiency. Drainage water salinity at the exit of each district was mostly affected by soil salinity and irrigation efficiency, with values very high in Tunisia (9.0 dS m⁻¹), high in Spain (4.6 dS m⁻¹), moderate in Morocco (estimated at 2.6 dS m⁻¹), and low in Turkey (1.4 dS m⁻¹). Salt loads in drainage waters, calculated from their salinity (EC_dw) and volume (Q), were highest in Tunisia (very high Q and very high EC_dw), intermediate in Turkey (extremely high Q and low EC_dw) and lowest in Spain (very low Q and high EC_dw) (there were no Q data for Morocco). Reduction of these high drainage volumes through sound irrigation management would be the most efficient way to control the off-site salt-pollution caused by these Mediterranean irrigation districts.     

Comparison of flooded and furrow-irrigated rice on clay

In some situations, potential water savings or relatively steep slopes make furrow irrigation a useful management practice for rice (Oryza sativa L.). Furrow-irrigated and flooded rice were compared in a field study conducted during three growing seasons: 1990, 1991, and 1992, at the University of Arkansas Northeast Research and Extension Center, Keiser, Ark., USA on a Sharkey silty clay soil. Excessive levee seepage greatly affected the water-use data for flooded rice production; however, there appeared to be potential for water savings on the Sharkey soil with furrow irrigation. Yields for flooded production consistently exceeded those for furrow-irrigated, with 3-year averages of 7.04, 6.02, and 5.88 Mg ha^-1 for flooded and two furrow-irrigated treatments, respectively. The yield difference appeared due to greater individual grain weight for the flooded treatment. Attempts to compensate for the yield reduction through additional nitrogen applications were unsuccessful. These results are consistent with findings of reduced rice grain yield associated with sprinkler irrigation. Furrow irrigation at an estimated 19 mm soil water deficit had a 3-year average of 11 kg ha^-1 of rice produced per mm of irrigation water applied.     

Predicting soil salinity in response to different irrigation practices, soil types and rainfall scenarios

A model was developed to predict rootzone salinity under different irrigation practices on different soil types, with similar rainfall but different monthly distributions. A rootzone daily water and salt balance was performed using eight scenarios: two soil types (coarse textured vs. fine textured), two multi-year series of actual rainfall data and two irrigation practices (surface with fixed number of irrigations and ET-based sprinkler irrigation). All factors influenced the mean electrical conductivity (EC) of the rootzone in the growing season (EC_eS): (i) Surface irrigation led to lower EC_eS than sprinkler irrigation; (ii) Winter-concentrated rainfall caused lower EC_eS than rainfall distributed uniformly throughout the year; and (iii) Coarser-textured soil usually resulted in lower EC_eS than the finer textured. The EC_eS was related to the total precipitation of the hydrologic year and to the annual leaching fraction (LF) but surprisingly not to the seasonal LF. In most cases, the model predicted lower EC_eS than the FAO steady-state approach. Therefore, considering these site-specific features could lead to lower leaching requirements and the safe use of higher salinity water.     

Water-use efficiency of sorghum and groundnut under traditional and current irrigation in the Gezira scheme, Sudan

In the Gezira irrigation scheme in central Sudan, serious symptoms of water waste have been identified in the last two decades, especially in sorghum and groundnut fields. To quantify losses, water-use efficiencies and related parameters were obtained for these two food crops under the traditional attended daytime water application and the newly evolved unattended continuous watering method. In this on-farm research, the neutron scattering method was used to determine the actual soil water deficits of the two crops. A simple Penman equation was used for approximating reference crop evapotranspiration and evaporation losses from standing water and wet soil surface. An updated approach using the Penman-Monteith equation was additionally applied. The study revealed wastage of irrigation water in both irrigation methods but at different rates and also differently for each crop. In the attended field, the average seasonal over-irrigation, which is the difference between average application depth Q and average soil moisture deficit SWD, was observed to range between 0.4 and 1.5 of SWD (0.3 and 0.6 of Q) and the corresponding values in the unattended field were 0.6 and 3.2 of SWD (0.4 and 0.8 of Q). Higher values are shown by the groundnut subplots, which crop also suffers from excess water, and by the drier year as well as in the unattended fields. A first approximation is given, still including readily available water at harvest, of minimum water requirements in attended watering for maximum yields. In the drier year, when more irrigation water was applied, an amount equal to 30-50% of these minimum water requirements was lost in evaporation from standing water/wet surface, which is the main unproductive water. More frequent land levelling aiming at minimum standing water in better attended irrigation and farm management (e.g. weeding) are priority measures proposed. The quantitative on-farm water waste determinations represent the innovative content of this paper. Knowing precisely how large the problem is and being able to quantify its components will contribute much to the arguments of those who wish to take the proposed measures.     

Long term use of saline water for irrigation

Use of saline drainage water in irrigated agriculture, as a means of its disposal, was evaluated on a 60 ha site on the west side of the San Joaquin Valley. In the drip irrigation treatments, 50 to 59% of the irrigation water applied during the six-year rotation was saline with an EC_w ranging from 7 to 8 dS/m, and containing 5 to 7 mg/L boron and 220 to 310 g/L total selenium. Low salinity water with an EC_w of 0.4 to 0.5 dS/m and B 0.4 mg/1 was used to irrigate the furrow plots from 1982 to 1985 after which a blend of good quality water and saline drainage water was used. A six-year rotation of cotton, cotton, cotton, wheat, sugar beet and cotton was used. While the cotton and sugar beet yields were not affected during the initial six years, the levels of boron (B) in the soil became quite high and were accumulated in plant tissue to near toxic levels. During the six year period, for treatments surface irrigated with saline drainage water or a blend of saline and low salinity water, the B concentration in the soil increased throughout the 1.5 m soil profile while the electrical conductivity (EC_e) increased primarily in the upper l m of the profile. Increaszs in soil EC_e during the entire rotation occurred on plots where minimal leaching was practiced. Potential problems with germination and seedling establishment associated with increased surface soil salinity were avoided by leaching with rainfall and low-salinity pre-plant irrigations of 150 mm or more. Accumulation of boron and selenium poses a major threat to the sustainability of agriculture if drainage volumes are to be reduced by using drainage water for irrigation. This is particularly true in areas where toxic materials (salt, boron, other toxic minor elements) cannot be removed from the irrigated area. Continual storage within the root zone of the cropped soil is not sustainable.     

Determining percolation losses of packed clay soil from tensiometer data

The measurement or prediction of percolation losses in field situations is of great practical significance for efficient irrigation and for determination of the leaching requirement, particularly of clayey soils where impeded percolation occurs. Hydraulic properties and water losses in packed Ashutia clay soil were determined under prevented-evaporation and free-evaporation conditions using lysimeter and tensiometric techniques. Hydraulic conductivity was determined as a function of soil moisture content using percolation flux computed. An exponential relationship between hydraulic conductivity and soil water content K = ae^bθ, was found. The percolation and evaporation-plus-percolation fluxes estimated from tensiometer readings under prevented-and free-evaporation conditions, respectively, matched with profile water losses from lysimeter measurements. The error ranged between 0.01 and 0.82 mm day⁻¹ with high correlation coefficient indicating that water loss from a soil profile can be estimated from tensiometer readings.