Seasonal variation in groundwater levels and quality under intensively drained and grazed pastures in the Montagu catchment, NW Tasmania

Water quality is a significant environmental issue in the Montagu River and its estuary in north-west Tasmania. Groundwater is the major contributor to baseflow for about half of the year. ‘Hump and hollow’ surface drainage is increasingly being used to reduce the effects of seasonal waterlogging on pasture production. However, little is known about the effects of ‘hump and hollow’ structures on watertable levels or intensive grazing on groundwater quality in the catchment. The objectives of this study were to evaluate the impacts of ‘hump and hollow’ drainage by comparing watertable levels in drained and undrained paddocks and to quantify the effects of intensive grazing on groundwater quality underlying pastures.In December 2004, 10 wells and 2 piezometers were installed at depths of 2-6 m at seven sites along two transects across the dairying area of Togari. Water levels were monitored and water samples collected every 2 months were analysed for pH, electrical conductivity, total dissolved solids, ammonium, nitrate, nitrite, total nitrogen, dissolved reactive phosphorous, Ca, Mg, K and Na. Thermotolerant coliforms and Enterococcus were measured when watertable levels were low and high.Watertable levels were within 0.5 m of ground level for over 3 months on undrained sites. ‘Hump and hollow’ surface drainage increased the depth of the unsaturated zone under the ‘humps’ but did not lower the watertable. Watertable levels on the crests of the ‘hump and hollow’ structures rose and fell daily in response to periods of rainfall and drought. Gradients of the groundwater surface, albeit very low, indicated the potential for groundwater flow from the base of the hills to the Montagu River in the centre of the valley.The median nitrate concentration of all samples was 0.018 mg NO₃-N L⁻¹ but one site had nitrate concentrations in excess of that recommended for potable water for a period of 1-2 months. Nitrate concentrations varied seasonally by 20-1000 times with an early winter pulse of nitrate evident both in the groundwater and in the Montagu River. In contrast, the median ammonium concentration in the groundwater was 0.274 mg NH₄-N L⁻¹ which was well above the trigger value for lowland streams. The median concentration of dissolved reactive phosphorus was 0.008 mg P L⁻¹ which was slightly higher than the trigger value. There was some evidence of low levels of faecal bacterial contamination of the shallow aquifers.Transects across the dairying area did not clearly demonstrate increasing concentrations of analytes due to intensive grazing though lower levels of nutrients were generally found at sites adjacent to undisturbed native forest. Variation in water quality parameters along the transects suggested water quality at a site was mostly related to local conditions and hazards.     

Micro-catchment water harvesting potential of an arid environment

Micro-catchment water harvesting (MCWH) requires development of small structures across mild land slopes, which capture overland flow and store it in soil profile for subsequent plant uses. Water availability to plants depends on the micro-catchment runoff yield and water storage capacity of both the plant basin and the soil profile in the plant root zone. This study assessed the MCWH potential of a Mediterranean arid environment by using runoff micro-catchment and soil water balance approaches. Average annual rainfall and evapotranspiration of the studied environment were estimated as 111 and 1671 mm, respectively. This environment hardly supports vegetation without supplementary water. During the study period, the annual rain was 158 mm in year 2004/2005, 45 mm in year 2005/2006 and 127 mm in year 2006/2007. About 5000 MCWH basins were developed for shrub raising on a land slope between 2 and 5% by using three different techniques. Runoff at the outlets of 26 micro-catchments with catchment areas between 13 and 50 m² was measured. Also the runoff was indirectly assessed for another 40 micro-catchments by using soil water balance in the micro-catchments and the plant basins. Results show that runoff yield varied between 5 and 187 m³ ha⁻¹ for various rainfall events. It was between 5 and 85% of the incidental rainfall with an average value of 30%. The rainfall threshold for runoff generation was estimated about 4 mm. Overall; the soil water balance approach predicted 38-57% less water than micro-catchment runoff approach. This difference was due to the reason that the micro-catchment runoff approach accounted for entire event runoff in the tanks; thus showed a maximum water harvesting potential of the micro-catchments. Soil water balance approach estimated water storage in soil profile and did not incorporate water losses through spillage from plant basins and deep percolation. Therefore, this method depicted water storage capacity of the plant basins and the root zone soil profile. The different between maximum water harvesting potential and soil-water storage capacity is surplus runoff that can be better utilized through appropriate MCWH planning.     

Effects of rising atmospheric CO2 on crop evapotranspiration in a Mediterranean area

In the assessment of plant response to the climate changes, the effects of CO₂ increase in the atmosphere and the subsequent rise of temperatures must be taken into account for their effects on crop physiology. In Mediterranean areas, a decrease of water availability and a more frequent occurrence of drought periods are expected. The objective of this study was to assess the impact of elevated CO₂ concentration and high temperature on reference evapotranspiration (ETo) and crop evapotranspiration (ETc) in the Mediterranean areas. The Penman-Monteith equation was used to simulate the future changes of reference evapotranspiration (ETo) by the recalibration of the canopy resistance parameter. Besides, crop coefficients (Kc) were adjusted according to the future climate trend. Then the modified empirical model (ETc = ETo × Kc) was applied providing an effective quantification of the climate change impact on water use of irrigated crops grown in Mediterranean areas. In the studied area, water use assessment was carried out for the period from 1961 to 2006 (measured data) and for a period from 2071 until 2100 (simulated data), showing a future climatic scenario. Water and irrigation use of crops will change as a function of climate changes, thermal needs of single crops and time of the year when they grow. Climate simulation model foresees the tendency for a significant increase of temperatures and a decrease of total year rainfall with a change of their distribution. The temperature increase and the concomitant expected rainfall decrease lead to a rise of year potential water deficit. About the autumn-spring crops, as wheat, a further increase of water deficit, is not expected. On the contrary, for spring-summer crops as tomato, a significant increase of water deficit and thus of irrigation need, is foreseen. Actually, for crops growing in that period of the year, the substantial rise of evapotranspiration demand cannot be compensated by crop cycle reduction and partial stomatal closure.     

Simulation of nitrate leaching under irrigated maize on sandy soil in desert oasis in Inner Mongolia, China

Water scarcity and nitrate contamination in groundwater are serious problems in desert oases in Northwest China. Field and ¹⁵N microplot experiments with traditional and improved water and nitrogen management were conducted in a desert oasis in Inner Mongolia Autonomous Region. Water movement, nitrogen transport and crop growth were simulated by the soil-plant system with water and solute transport model (SPWS). The model simulation results, including the water content and nitrate concentration in the soil profile, leaf area index, dry matter weight, crop N uptake and grain yield, were all in good agreement with the field measurements. The water and nitrogen use efficiency of the improved treatment were better than those of the traditional treatment. The water and nitrogen use efficiency under the traditional treatment were 2.0 kg m⁻³ and 21 kg kg⁻¹, respectively, while under the improved treatment, they were 2.2 kg m⁻³ and 26 kg kg⁻¹, respectively. Water drainage accounted for 24-35% of total water input (rainfall and irrigation) for the two treatments. Nitrogen loss by ammonia volatilization and denitrification was less than 5% of the total N input (including the N comes from irrigation). However, 32-61% of total nitrogen input was lost through nitrate leaching, which agreed with the ¹⁵N isotopic result. It is impetrative to improve the water and nitrogen management in the desert oasis.     

Annual carbon and nitrogen loadings for a furrow-irrigated field

Evaluations of agricultural management practices for soil C sequestration have largely focused on practices, such as reduced tillage or compost/manure applications, that minimize soil respiration and/or maximize C input, thereby enhancing soil C stabilization. Other management practices that impact carbon cycling in agricultural systems, such as irrigation, are much less understood. As part of a larger C sequestration project that focused on potential of C sequestration for standard and minimum tillage systems of irrigated crops, the effects of furrow irrigation on the field C and N loading were evaluated. Experiments were conducted on a laser-leveled 30 ha grower's field in the Sacramento valley near Winters, CA. For the 2005 calendar year, water inflow and runoff was measured for all rainfall and irrigation events. Samples were analyzed for C and N associated with both sediment and dissolved fractions. Total C and N loads in the sediment were always higher in the incoming irrigation water than field runoff. Winter storms moved little sediment, but removed substantial amounts of dissolved organic carbon (DOC), or about one-third of the total C balance. Despite high DOC loads in runoff, the large volumes of applied irrigation water with sediment and DOC resulted in a net increase in total C for most irrigation events. The combined net C input and N loss to the field, as computed from the field water balance, was 30.8 kg C ha⁻¹ yr⁻¹ and 5.4 kg N ha⁻¹ yr⁻¹ for the 2005 calendar year. It is concluded that transport of C and N by irrigation and runoff water should be considered when estimating the annual C field balance and sequestration potential of irrigated agro-ecosystems.     

Yield and fruit development in mango (Mangifera indica L. cv. Chok Anan) under different irrigation regimes

‘Chok Anan’ mangoes are mainly produced in the northern part of Thailand for the domestic fresh market and small scale processing. It is appreciated for its light to bright yellow color and its sweet taste. Most of the fruit development of on-season mango fruits takes place during the dry season and farmers have to irrigate mango trees to ensure high yields and good quality. Meanwhile, climate changes and expanding land use in horticulture have increased the pressure on water resources. Therefore research aims on the development of crop specific and water-saving irrigation techniques without detrimentally affecting crop productivity.The aim of this study was to assess the response of mango trees to varying amounts of available water. Influence of irrigation, rainfall, fruit set, retention rate and alternate bearing were considered as the fruit yield varies considerably during the growing seasons. Yield response and fruit size distribution were measured and WUE was determined for partial rootzone drying (PRD), regulated deficit irrigation (RDI) and irrigated control trees.One hundred ninety-six mango trees were organized in a randomized block design consisting of four repetitive blocks, subdivided into eight fields. Four irrigation treatments have been evaluated with respect to mango yield and fruit quality: (a) control (CO = 100% of ET_c), (b) (RDI = 50% of ET_c), (c) (PRD = 50% of ET_c, applied to alternating sides of the root system) and (d) no irrigation (NI).Over four years, the average yield in the different irrigation treatments was 83.35 kg/tree (CO), 80.16 kg/tree (RDI), 80.85 kg/tree (PRD) and 66.1 kg/tree (NI). Water use efficiency (WUE) calculated as yield per volume of irrigation water was always significantly higher in the deficit irrigation treatments as compared to the control. It turned out that in normal years the yields of the two deficit irrigation treatments (RDI and PRD) do not differ significantly, while in a dry year yield under PRD is higher than under RDI and in a year with early rainfall, RDI yields more than PRD. In all years PRD irrigated mangoes had a bigger average fruit size and a more favorable fruit size distribution.It was concluded that deficit irrigation strategies can save considerable amounts of water without affecting the yield to a large extend, possibly increasing the average fruit weight, apparently without negative long term effects.     

Ameliorating water repellency under turfgrass of contrasting soil organic matter content: Effect of wetting agent formulation and application frequency

Water conservation strategies are being developed in regions of the world expected to experience decreases in water resources due to changing climates. Strategies advocated for improving water-use efficiency may increase the incidence of soil water repellency in sandy-textured soils. We evaluated the effect of soil wetting agent formulation, and application frequency, on water repellency in sandy soil with two contrasting organic matter (OM) contents under kikuyugrass [Pennisetum clandestinum (Holst. Ex Chiov)], and irrigated at 60% replacement of net evaporation in a climate subject to hot, dry summers. The randomized plot design included two turfgrass ages [established from 20 week (7.7% OM) or 20 year old (30% OM) turfgrass in 2005, the latter included a 50 mm ‘mat’ layer], two soil wetting agent formulations (granular or liquid); two application frequencies (one or two applications per irrigation season); and plots of both turfgrass ages that did not receive any wetting agent (nil control). Both wetting agent formulations contained the same active ingredient (propylene oxide-ethylene oxide block polymer), and all wetting agent treatments received the same rate (69 L active ingredient ha⁻¹). Water repellency in the surface soil (0-5 mm), measured using the molarity of ethanol droplet test (MED), ranged from 1.09 M to 4.32 M during the irrigation season, and was more severe in the soil with high OM (average MED, 3.3 M) than low OM content (average MED, 2.7 M). Applying one application of either granular or liquid soil wetting agent at the commencement of the irrigation season decreased the severity of soil water repellency by up to 30% in the high OM soil and by up to 60% in the low OM soil during the summer, and without the need for a second application. The decline in soil water repellency in response to soil wetting agent application was not matched by an increase in soil VWC in summer, and turfgrass quality was considered acceptable throughout the study. The soil wetting agents were less effective at treating water repellent sand containing a significant amount of OM than sand with low OM content.     

Cover crop evapotranspiration under semi-arid conditions using FAO dual crop coefficient method with water stress compensation

Cover cropping is a common agro-environmental tool for soil and groundwater protection. In water limited environments, knowledge about additional water extraction by cover crop plants compared to a bare soil is required for a sustainable management strategy. Estimates obtained by the FAO dual crop coefficient method, compared to water balance-based data of actual evapotranspiration, were used to assess the risk of soil water depletion by four cover crop species (phacelia, hairy vetch, rye, mustard) compared to a fallow control. A water stress compensation function was developed for this model to account for additional water uptake from deeper soil layers under dry conditions. The average deviation of modelled cumulative evapotranspiration from the measured values was 1.4% under wet conditions in 2004 and 6.7% under dry conditions in 2005. Water stress compensation was suggested for rye and mustard, improving substantially the model estimates. Dry conditions during full cover crop growth resulted in water losses exceeding fallow by a maximum of +15.8% for rye, while no substantially higher water losses to the atmosphere were found in case of evenly distributed rainfall during the plant vegetation period with evaporation and transpiration concentrated in the upper soil layer. Generally the potential of cover crop induced water storage depletion was limited due to the low evaporative demand when plants achieved maximum growth. These results in a transpiration efficiency being highest for phacelia (5.1 g m⁻² mm⁻¹) and vetch (5.4 g m⁻² mm⁻¹) and substantially lower for rye (2.9 g m⁻² mm⁻¹) and mustard (2.8 g m⁻² mm⁻¹). Taking into account total evapotranspiration losses, mustard performed substantially better. The integration of stress compensation into the FAO crop coefficient approach provided reliable estimates of water losses under dry conditions. Cover crop species reducing the high evaporation potential from a bare soil surface in late summer by a fast canopy coverage during early development stages were considered most suitable in a sustainable cover crop management for water limited environments.     

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

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

Seasonal variations in sap flow and soil evaporation in an olive (Olea europaea L.) grove under two irrigation regimes in an arid region of Argentina

The emergence of intensively managed olive plantations in arid, northwestern Argentina requires the efficient use of irrigation water. We evaluated whole tree daily transpiration and soil evaporation throughout the year to better understand the relative importance of these water use components and to calculate actual crop coefficient (Kc) values. Plots in a 7-year-old ‘Manzanilla fina’ olive grove with 23% canopy cover were either moderately (MI) or highly irrigated (HI) using the FAO method where potential evapotranspiration over grass is multiplied by a given Kc and a coefficient of reduction (Kr). The Kc values employed for the MI and HI treatments were 0.5 and 1.1, respectively, and the Kr was 0.46. Transpiration was estimated by measuring main trunk sap flow using the heat balance method for three trees per treatment. Soil evaporation was measured using six microlysimeters in one plot per treatment. Both parameters were evaluated for 7-10 consecutive days in the fall, winter, mid-spring, summer, and early fall of 2006-2007. Maximum soil evaporation was observed in the summer when maximum demand was combined with maximum surface wetted by the drips and evaporation from the inter-row occurred due to rainfall. Similarly, maximum daily transpiration was observed in mid-spring and summer. Transpiration of MI trees was 30% lower than in HI trees during the summer period. However, this difference in transpiration disappeared when values were adjusted for total leaf area per tree because leaf area was 28% less in the MI trees. Transpiration represented about 70-80% of total crop evapotranspiration (ETc) except when soil evaporation increased due to rainfall events or over-irrigation occurred. We found that daily transpiration per unit leaf area had a positive linear relationship with daily potential evapotranspiration (r² = 0.84) when considering both treatments together. But, a strong relationship was also observed between transpiration per unit leaf area and mean air temperature (r² = 0.93). Thus, it is possible to predict optimum irrigation requirements for olive groves if tree leaf area and temperature are known. Calculated crop coefficients during the growing season based on the transpiration and soil evaporation values were about 0.65-0.70 and 0.85-0.90 for the MI and HI treatments, respectively.     

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.     

Unrestricted cattle access to streams and water quality in till landscape of the Midwest

Unrestricted cattle access to streams in traditionally pastoral regions has been linked to increased concentrations of bacteria, suspended sediments and associated contaminants in streams. However, there is a dearth of data available regarding the impact of cattle access to streams in poorly drained landscapes of the Midwest. In this study, we investigate changes in water quality on a 1005 m long stream section impacted by cattle grazing on the upper 130 m. We monitor discharge, water quality [nitrate, ammonium, total Kjeldahl nitrogen (TKN), total phosphorus (TP), total suspended sediments (TSS), turbidity, Escherichia coli] and chloride, atrazine, silica and major cation concentrations over a 12-month period. Cattle access to the stream does not significantly affect nitrate concentration but leads to large increases in TKN (fourfold increase), TP (fivefold increase), ammonium (fourfold increase), TSS (11-fold increase), turbidity (13-fold increase) and E. coli (36-fold increase) in the summer/fall period. In particular, E. coli concentration in the stream in the summer/fall period exceeds 235 colony forming unit (CFU)/100 ml 64% of the time upstream from the section impacted by cattle, but exceeds this same threshold 89% of the time immediately downstream. Despite the negative impact of cattle access to the stream on water quality, data indicate that dilution, in-stream processes, and natural stream geometry downstream from the impacted section help mitigate this pollution. We expect that this study will be an incentive for policy makers to promote stream rehabilitation and develop more stringent guidelines limiting cattle access to streams in many Midwestern states and other regions with poorly drained soils where the impact of cattle access to streams on water quality is often ignored.     

Effects of winter wheat row spacing on evapotranpsiration, grain yield and water use efficiency

A field study was conducted from 2002 to 2007 to investigate the influence of row spacing of winter wheat (Triticum aestivum L.) on soil evaporation (E), evapotranspiration (ET), grain production and water use efficiency (WUE) in the North China Plain. The experiment had four row spacing treatments, 7.5 cm, 15 cm, 22.5 cm, and 30 cm, with plots randomly arranged in four replicates. Soil E was measured by micro-lysimeters in three seasons and ET was calculated from measurements of soil profile water depletion, irrigation, and rainfall. The results showed that E increased with row spacing. Compared with the 30-cm row spacing (average E = 112 mm), the reduction in seasonal E averaged 9 mm, 25 mm, and 26 mm for 22.5 cm, 15 cm, and 7.5 cm row spacings, respectively. Crop transpiration (T) increased as row spacing decreased. The seasonal rainfall interception and seasonal ET were relatively unchanged among the treatments. In three out of five seasons, the four different treatments showed similar grain yield, yield components and WUE. We conclude that for winter wheat production in the North China Plain, narrow row spacing reduced soil evaporation, but had minor improvements on grain production and WUE under irrigated conditions with adequate nutrient levels.     

Efficiency of controlled drainage and subirrigation in reducing nitrogen losses from agricultural fields

In northeast Italy, a regimen of controlled drainage in winter and subirrigation in summer was tested as a strategy for continuous water table management with the benefits of optimizing water use and reducing unnecessary drainage and nitrogen losses from agricultural fields.To study the feasibility and performance of water table management, an experimental facility was set up in 1996 to reproduce a hypothetical 6-ha agricultural basin with different land drainage systems existing in the region. Four treatments were compared: open ditches with free drainage and no irrigation (O), open ditches with controlled drainage and subirrigation (O-CI), subsurface corrugated drains with free drainage and no irrigation (S), subsurface corrugated drains with controlled drainage and subirrigation (S-CI). As typically in the region free drainage ditches were spaced 30 m apart, and subsurface corrugated drains were spaced 8 m apart.Data were collected from 1997 to 2003 on water table depth, drained volume, nitrate-nitrogen concentration in the drainage water, and nitrate-nitrogen concentration in the groundwater at various depths up to 3 m.Subsurface corrugated drains with free drainage (S) gave the highest measured drainage volume of the four regimes, discharging, on average, more than 50% of annual rainfall, the second-highest concentration of nitrate-nitrogen in the drainage water, and the highest nitrate-nitrogen losses at 236 k ha⁻¹.Open ditches with free drainage (O) showed 18% drainage return of rainfall, relatively low concentration of nitrate-nitrogen in the drainage water, the highest nitrate-nitrogen concentration in the shallow groundwater, and 51 kg ha⁻¹ nitrate-nitrogen losses.Both treatments with controlled drainage and subirrigation (O-CI and S-CI) showed annual rainfall drainage of approximately 10%. O-CI showed the lowest nitrate-nitrogen concentration in the drainage water, and the lowest nitrogen losses (15 kg ha⁻¹). S-CI showed the highest nitrate-nitrogen concentration in the drainage water, and 70 kg ha⁻¹ nitrate-nitrogen losses. Reduced drained volumes resulted from the combined effects of reduced peak flow and reduced number of days with drainage.A linear relationship between daily cumulative nitrate-nitrogen losses and daily cumulative drainage volumes was found, with slopes of 0.16, 0.12, 0.07, and 0.04 kg ha⁻¹ of nitrate-nitrogen lost per mm of drained water in S-CI, S, O, and O-CI respectively.These data suggest that controlled drainage and subirrigation can be applied at farm scale in northeast Italy, with advantages for water conservation.     

Effects of irrigation water salinity and electrostatic water treatment for sugarcane production

Excess salinity in irrigation water reduces sugarcane yield and juice quality. This study was conducted to compare the effect of irrigation with water of 1.3 dS m⁻¹ vs. 3.4 dS m⁻¹ on sugarcane yield and quality, and to evaluate whether an electrostatic conditioning treatment of the water influenced the salt effects. The study was conducted in a commercial field divided into large plots ranging from 1.0 to 1.2 ha in size. Cane and sugar yields were reduced approximately 17% by the 3.4 dS m⁻¹ water compared to the 1.3 dS m⁻¹ water, but juice quality parameters were not affected. Conditioning of the irrigation water using a device called an ‘electrostatic precipitator’ which claimed to affect various water properties had no effect on cane yield, juice quality or soil salinity levels. The detrimental effect of the high salt irrigation water was somewhat less than might be expected, probably due to good late summer rainfall which may have flushed the root zone from the excessive salts.     

Supplemental irrigation of wheat with saline water

In arid and semi-arid regions, both rainfall and surface irrigation water supplies are unreliable and inadequate to meet crop water requirement. Groundwater in these regions is mainly marginally saline (2-6 dS/m) to saline (>6 dS/m) and could be exploited to meet crop water requirement if no adverse effects on crops and land resource occur. The fear of adverse effects has often restricted the exploitation of naturally occurring saline water. The results reveal that substituting a part or all except pre-sowing irrigation with saline water having an electrical conductivity (EC_iw) of 8 dS/m is possible for cultivation of wheat. Similarly, saline water with EC_iw ranging between 8 and 12 dS/m could be used to supplement at least two irrigations to obtain 90% or more of the optimum yield. In low rainfall years, the use of such waters for all irrigations, except pre-sowing, produced more yield than skipping irrigations. Apparently, even at this level of osmotic salt stress, matric stress is more harmful. Thus, it would be interesting to use such waters for wheat production in monsoon climatic regions.     

Water management options based on rainfall analysis for rainfed maize (Zea mays L.) production in Rushinga district, Zimbabwe

Maize (Zea mays L.), the dominant and staple food crop in Southern and Eastern Africa, is preferred to the drought-tolerant sorghum and pearl millet even in semi-arid areas. In semi-arid areas production of maize is constrained by droughts and poor rainfall distribution. The best way to grow crops in these areas is through irrigation, but limited areal extent, increasing water scarcity, and prohibitive development costs limit the feasibility of irrigation. Therefore, there is need for a policy shift towards other viable options. This paper presents daily rainfall analysis from Rushinga district, a semi-arid location in Northern Zimbabwe. The purpose of the rainfall analysis was to assess opportunities and limitations for rainfed maize production using 25 years of data. Data was analysed using a variety of statistical methods that include trend analysis, t-test for independent samples, rank-based frequency analysis, Spearman's correlation coefficient and Mann-Whitney's U test. The results showed no evidence of change in rainfall pattern. The mean seasonal rainfall was 631 mm with a standard deviation (SD) of 175 mm. December, January and February consistently remained the major rainfall months. The results depicted high inter-annual variability for both annual and seasonal rainfall totals, a high incidence of droughts ≥3 out of every 10 years and ≥1 wet year in 10 years. Using the planting criteria recommended in Zimbabwe, most of the plantings would occur from the third decade of November with the mode being the first decade of December. This predisposes the rainfall to high evaporation and runoff losses especially in December when the crop is still in its initial stage of growth. On average 5 to more than 20 days dry spells occupy 56% of the rainy season. Seasonal rainfall exhibited negative correlation (P < 0.001; R = −0.746) with cumulative dry spell length, and wet years were free from dry spells exceeding 20 days. The most common dry spells (6-10 days), are in the range in which irrigated crops survive on available soil water. Therefore, they can be mitigated by in situ rainwater harvesting (RWH) and water conservation. The potential evapotranspiration of a 140-day maize crop was estimated to be 540 mm. Consequently, short season maize cultivars that mature in less than 140 days could be grown successfully in this area in all but drought years. However, sustainable maize production can only be achieved with careful management of the soil as a medium for storing water, which is essential for buffering against dry spells. To this end soil restorative farming systems are recommended such as conservation farming, in situ RWH techniques for dry spell mitigation and a cropping system that includes drought-tolerant cereal crops as for example sorghum and pearl millet, and perennial carbohydrate sources as for example cassava to provide stable crop yields.     

Hydro-physical characteristics of selected media used for containerized agriculture systems

Containerized plant production represents an extremely intensive agricultural practice with large amounts of moisture and fertilizer application. Hydro-physical characteristics such as water infiltration, texture and structure, particle size distribution affect the quality of the media used in containerized agricultural systems and the water availability to plants. Water retention characteristics depend on particle size distribution as well as the composition of the media used. Materials with coarser particles allow faster percolation of water and also retain relatively higher amounts moisture per unit weight due to higher porosity, while draining faster due to smaller surface area per unit weight. Faster drainage can result into airflow through coarser materials causing the media to dry. The objectives of this study were to characterize the selected hydro-physical properties of plant growth media that are commonly used by nurseries in South Florida. Characterization of the plant growing media can allow modeling of soil-water interactions and development of best management practices for more efficient use of water and agrochemicals by nurseries. Experimental analyses were performed to characterize the plant growth mixtures in terms of particle size distribution and hydraulic conductivity using three different methods (i.e., constant head permeability, falling head permeability test, and tension infiltrometer test). The saturated hydraulic conductivity of the mixtures measured by constant head method ranged from 0.029 to 0.042 cm/s (104-151 cm/h) and by falling head method ranged from 0.078 to 0.112 cm/s (281-403 cm/h). The saturated hydraulic conductivity of the mixtures measured by tension infiltrometer ranged from 0.02 to 0.34 cm/h. Understanding water retention and permeation characteristics of the plant growing media could assist development of best management practices (BMP) for containerized agricultural systems for efficient management of irrigation water and agrochemical use.     

Evaporative water use of different land uses in the upper-Thukela river basin assessed from satellite imagery

Evaporative water use of various land use classes in the upper-Thukela river basin was estimated using the public domain version of the Surface Energy Balance Algorithm for Land (SEBAL) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images. Twenty eight images were analyzed covering the period between June 2005 and September 2006. The South Africa land use map developed in the year 2000 was used to compute the evaporative water use of the various land uses in 13 Quaternary Catchments (QCs) in the upper-Thukela river basin.There was a good correlation between the SEBAL estimates of total evaporation and ground measurements from a Large Aperture Scintillometer installed at a site in one of the QCs in the study area. It was observed that the land uses that generated relatively large volumes of evaporative water were forestry (i.e. Eucalyptus, Pine, mixed species & indigenous), “water bodies” (i.e. water supply reservoirs, farm reservoirs) and wetlands. Total evaporation rates for all land use classes were high during the summer season (wet), with Eucalyptus ranging between 3 mm d⁻¹ during the winter season (dry) and about 5 mm d⁻¹ during the summer season. Bare rocks and eroded soil surface land use class had the least evaporative water use i.e., less than 1 mm d⁻¹. The relatively low evaporation rates over “water bodies” during the dry winter season could be attributed to the averaging of mixed pixels (wet and dry) due to change in areal coverage and which could not be captured in the 1 km × 1 km low resolution MODIS images. Thus, the potential for applying remote sensing techniques, using low resolution satellite images, to quantify water use by various land uses in the Thukela river basin was explored with promising results.     

Irrigation strategies to improve the water use efficiency of wheat-maize double cropping systems in North China Plain

Water is the most important limiting factor of wheat (Triticum aestivum L.) and maize (Zea mays L.) double cropping systems in the North China Plain (NCP). A two-year experiment with four irrigation levels based on crop growth stages was used to calibrate and validate RZWQM2, a hybrid model that combines the Root Zone Water Quality Model (RZWQM) and DSSAT4.0. The calibrated model was then used to investigate various irrigation strategies for high yield and water use efficiency (WUE) using weather data from 1961 to 1999. The model simulated soil moisture, crop yield, above-ground biomass and WUE in responses to irrigation schedules well, with root mean square errors (RMSEs) of 0.029 cm³ cm⁻³, 0.59 Mg ha⁻¹, 2.05 Mg ha⁻¹, and 0.19 kg m⁻³, respectively, for wheat; and 0.027 cm³ cm⁻³, 0.71 Mg ha⁻¹, 1.51 Mg ha⁻¹ and 0.35 kg m⁻³, respectively, for maize. WUE increased with the amount of irrigation applied during the dry growing season of 2001-2002, but was less sensitive to irrigation during the wet season of 2002-2003. Long-term simulation using weather data from 1961 to 1999 showed that initial soil water at planting was adequate (at 82% of crop available water) for wheat establishment due to the high rainfall during the previous maize season. Preseason irrigation for wheat commonly practiced by local farmers should be postponed to the most sensitive growth stage (stem extension) for higher yield and WUE in the area. Preseason irrigation for maize is needed in 40% of the years. With limited irrigation available (100, 150, 200, or 250 mm per year), 80% of the water allocated to the critical wheat growth stages and 20% applied at maize planting achieved the highest WUE and the least water drainage overall for the two crops.