Seasonal water use by winter wheat grown under shallow water table conditions

Techniques for estimating seasonal water use from soil profile water depletion frequently do not account for flux below the root zone. A method using tensiometers for obtaining evapotranspiration losses from the root zone and water movement below it is discussed. Soil water flux below the root zone is approached by a sequence of pseudo steady state solutions of the flow equation. Upward soil water flux contributed 36 to 73% to the total water requirement of winter wheat (Triticum aestivum L.) whereas soil water depletion accounted for 11 to 19% only. Water use efficiency with one irrigation during an early stage of plant development is greater than with no or three irrigations. This is the result of both decrease of resistance due to soil moistening and better root development. Tensiometer readings were also interpreted to estimate root zones, water table depths and soil moisture contents. Methods described in this paper can be used in determining seasonal water use by growing crops, replacing or supplementing lysimeter or meteorology approaches to this problem.     

In situ water transmission characteristics of a tropical soil under rice-based cropping systems

In soils under rice-based cropping systems in Asia water movement and distribution in the root zone of rice and dryland crops are important for efficient water management. Saturated hydraulic conductivities in the wetland soil profile were evaluated from measurements of hydraulic gradients and percolation rates in the field. The subsoil layer (15–60 cm) restricted percolation rate to a greater degree than the puddled top soil.Unsaturated hydraulic conductivities and soil water diffusivities in the soil profile under dryland conditions were obtained from simultaneous measurements of soil water content using the neutron moderation technique and the soil matric potential by tensiometers over time and soil depth. Soil matric potential versus hydraulic conductivity and soil water content versus soil water diffusivity relations of various soil depths were established. At equivalent soil matric potentials, the hydraulic conductivity of surface soil was greater than that of the subsoil layers. Soil water diffusivity at different depths responded similarly. The study describes a simple in situ technique to measure percolation rates in wetland rice fields and evaluation of water transmission properties of field soil profiles.     

Nitrate leaching and soil nitrate content as affected by irrigation uniformity in a carrot field

High value crops such as carrot planted in coarse soils of the Southern San Joaquin Valley in California are prime candidates for nitrate leaching through irrigation nonuniformity. A 2-year study was carried out to explore the impact of irrigation uniformity on nitrate leaching. Irrigation uniformity was measured using catchcans. Soil nitrate (NO₃-N) and ammonium (NH₄-N) contents were measured from soil sampled at different depths and times during two growing seasons. Nitrate leaching was determined using ion-exchange resin bags at 1-m depth sampled three times during each season. Although, soil NO₃-N as well as seasonal irrigation was significantly higher along the lateral irrigation pipe than between the sprinklers, nitrate leaching was not significantly higher. As expected, soil nitrate content decreased as percolation increased for both years. Nitrate leaching, as estimated by anion-exchange resin bags, was positively correlated to soil NO₃-N content but was not correlated to irrigation depth, irrigation uniformity, or deep percolation. Field variation in saturated hydraulic conductivity (K_s), soil organic matter (OM), and soil water retention at field capacity had limited effect on NO₃-N and NH₄-N distributions in the profile and on nitrate leaching. The results of this experiment suggest that irrigation nonuniformity has less impact on nitrate movement than suggested by earlier studies.     

Evapotranspiration of citrus as affected by soil water deficit and soil salinity

Summary The extent to which evapotranspiration (ET) of Valencia citrus trees is affected by differing soil water depletions (SWD) and soil salinity regimes was determined during five seasons during which soil salinity levels varied. Three weighing lysimeters, each with a 14 year old tree, were used to measure daily ET and to schedule irrigation to maintain SWD at maxima of 15, 75 and 150 mm respectively. Tensiometers and salinity sensors were used to indicate the in situ soil matric and soil solution osmotic potentials. Total soil water potential was calculated from tensiometer and salinity sensor readings weighted for root density with depth. The total of these for the summer months was found to be linearly related (Fig. 5) to the mean ET/Ep (Ep=A-pan evaporation). The slope and threshold of ET reductions with decreasing soil water potential for the low frequency irrigation treatment (150 mm SWD) show good agreement with the slope and threshold of yield decrease that is calculated from soil salinity in the lysimeter using previously reported salinity-yield relationships. The reduced water uptake due to increasing soil salinity has important implications for soil salinity control, since the lower uptake should in theory increase the leaching fraction. This implies a degree of self adjustment to the leaching fraction when irrigating with increasingly saline waters if water applications are scheduled as for non-saline conditions.     

Effect of mixed cation solutions on hydraulic soil properties

Hydraulic conductivity (K) and soil water diffusivity (D) characterizing water flow under saturated and unsaturated conditions, respectively, were determined for a sandy loam and a clay loam soil, using water with different combinations of total electrolyte concentrations, C (i.e., 20, 40, 80, 125 and 250 meq 1^?1) and sodium adsorption ratios, SAR (i.e., 0, 20, 30, 40, 80 and ∞ mmole $\text{l}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$). Both K and D were found to increase with C and decrease with SAR. In low sodium adsorption ratio ranges (i.e., up to 20) the requirement of electrolyte concentration to maintain relative hydraulic conductivity = 0.5 was relatively more for sandy loam than for clay loam soil. However, the trend for electrolyte concentration requirements for the two soils was reversed at high sodium adsorption ratios (i.e. > 20). A spline function was used to draw the best fitting line through the data points of horizontal absorption experiments.     

Consumptive water use and crop coefficients of irrigated sunflower

In semi-arid environments, the use of irrigation is necessary for sunflower production to reach its maximum potential. The aim of this study was to quantify the consumptive water use and crop coefficients of irrigated sunflower (Helianthus annuus L.) without soil water limitations during two growing seasons. The experimental work was conducted in the lysimeter facilities located in Albacete (Central Spain). A weighing lysimeter with an overall resolution of 250 g was used to measure the daily sunflower evapotranspiration throughout the growing season under sprinkler irrigation. The lysimeter container was 2.3 m × 2.7 m × 1.7 m deep, with an approximate total weight of 14.5 Mg. Daily ET _c values were calculated as the difference between lysimeter mass losses and lysimeter mass gains divided by the lysimeter area. In the lysimeter, sprinkler irrigation was applied to replace cumulative ET _c, thus maintaining non-limiting soil water conditions. Seasonal lysimeter ET _c was 619 mm in 2009 and 576 mm in 2011. The higher ET _c value in 2009 was due to earlier planting and a longer growing season with the maximum cover coinciding with the maximum ET _o period. For the two study years, maximum average K _c values reached values of approximately 1.10 and 1.20, respectively, during mid-season stage and coincided with maximum ground cover values of 75 and 88 %, respectively. The dual crop coefficient approach was used to separate crop transpiration (K _cb) from soil evaporation (K _e). As the crop canopy expanded, K _cb values increased while the K _e values decreased. The seasonal evaporation component was estimated to be about 25 % of ET _c. Linear relationships were found between the lysimeter K _cb and the canopy ground cover (f _c) for the each season, and a single relationship that related K _cb to growing degree-days was established allowing extrapolation of our results to other environments.     

Uptake of shallow groundwater by cotton: growth stage,groundwater salinity effects in column lysimeters

A 3-year column lysimeter experiment was conducted with cotton (Gossypium hirsutum L.) to determine the influence of shallow groundwater salinity on groundwater uptake. Nonsaline (0.3 dS m⁻¹) irrigation water was applied at 7-day intervals throughout the growing season, with the cotton allowed to use stored soil water and groundwater as root water uptake permitted. Groundwater salinities ranging from 0.3 dS m⁻¹ electrical conductivity (EC_w) to 30.8 dS m⁻¹ were evaluated. Water for leaching was applied following harvest each year in amounts adequate to produce a nonsaline soil profile at the beginning of each year. Equations were developed to describe relationships between day of year, growth stage or growing degree days and shallow groundwater uptake. Groundwater contributed about 30 to 42% of seasonal total evapotranspiration (ET) in treatments with groundwater salinity ≤ 20 dS m⁻¹ but declined to 12 to 19% of total ET at higher salinity levels.     

Soil water balance trial involving capacitance and neutron probe measurements

The objective of this study was to compare soil water measurements made using capacitance and neutron probes by means of a water balance experiment in a drainage lysimeter. The experiment was conducted in a 5-year-old drip-irrigated peach orchard (Prunus persica L. Batsch, cv. Flordastar, on GF-677 peach rootstock) planted in a clay loam textured soil located in southern Spain. Four drainage lysimeters (5 m × 5 m × 1.5 m), each containing one tree, were constructed and equipped with one lateral line containing eight drippers per tree, with a discharge rate of 2 L h⁻¹. Three access tubes for the neutron probe (NP), symmetrically facing three PVC access tubes containing the multi-depth capacitance probes (MDCP) were located perpendicularly to the drip line (0.2, 0.6 and 1 m). The results demonstrated that both the capacitance and neutron probes gave similar soil water content values under steady state hydraulic gradient conditions (0.2 m from the emitter) although some discrepancies were found in heterogeneous soil water distribution conditions (1 m from the emitter), which might be attributed to the smaller soil volume explored by the MDCP compared with the NP. Explanations for the discrepancies between both devised are presented. When water inputs and outputs were fairly constant, the volumetric soil water content could be considered to represent field saturation (θ_sat = 0.36 m³ m⁻³). When drainage was zero, there were 2 days when the soil water content was constant and could be considered as field capacity (θ_fc = 0.31 m³ m⁻³). The findings suggest that: (i) capacitance probes can be used for continuous real-time soil water content monitoring unlike the manual measurements obtained with the neutron probe; (ii) the location of the sensors is critical when used for drip irrigation scheduling and our recommendations for practical agricultural purposes would be to place MDCP sensors in the place representing the highest root density, leading the sensors to become biological sensors rather than mere soil moisture sensors; and (iii) on average, the water balance values determined by lysimeter match those calculated using the data from both probes. However, due to the smaller soil volume explored by MDCP, more of these sensors must be used to characterize the soil water status in water balance studies.     

Monitoring and modelling draining and resident soil water nitrate concentrations to estimate leaching losses

Quantifying nitrogen (N) losses below the root zone is highly challenging due to uncertainties associated with estimating drainage fluxes and solute concentrations in the leachate. Active and passive soil water samplers provide solute concentrations but give limited information on water fluxes. Mechanistic models are used to estimate leaching, but require calibration with measured data to ensure their reliability. Data from a drainage lysimeter trial under irrigation in which soil profile nitrate (NO₃⁻) concentrations were monitored using wetting front detectors (passive sampler) and ceramic suction cups (active sampler) were compared to NO₃⁻ concentrations in draining and resident soil water as simulated by the research version of the Soil Water Balance model (SWB-Sci). SWB-Sci is a daily time-step, cascading soil water and solute balance model that provides draining NO₃⁻ concentrations by accounting for incomplete solute mixing. As hypothesized, suction cup concentrations aligned closely with resident soil water concentrations, while wetting front detector concentrations aligned closely with draining soil water NO₃⁻ concentrations. These results demonstrate the power of combining monitoring and modelling to estimate NO₃⁻ leaching losses. Access to measured draining and resident NO₃⁻ concentrations, especially when complemented with modelled fluxes, can contribute greatly to achieving improved production and environmental objectives.     

Performance evaluation and calibration of soil water content and potential sensors for agricultural soils in eastern Colorado

This study evaluated the performance of three soil water content sensors (CS616/625, Campbell Scientific, Inc., Logan, UT; TDT, Acclima, Inc., Meridian, ID; 5TE, Decagon Devices, Inc., Pullman, WA) and a soil water potential sensor (Watermark 200SS, Irrometer Company, Inc., Riverside, CA) in laboratory and field conditions. Soil water content/potential values measured by the sensors were compared with corresponding volumetric water content (θ_v, m³ m⁻³) values derived from gravimetric samples, ranging approximately from the permanent wilting point (PWP) to field capacity (FC) volumetric water contents. Under laboratory and field conditions, the factory-based calibrations of θ_v did not consistently achieve the required accuracy for any sensor in the sandy clay loam, loamy sand, and clay loam soils of eastern Colorado. Salt (calcium chloride dihydrate) added to the soils in the laboratory caused the CS616, TDT, and 5TE sensors to experience errors in their volumetric water content readings with increased bulk soil electrical conductivity (EC; dS m⁻¹). Results from field tests in sandy clay loam and loamy sand soils indicated that a linear calibration (equations provided) for the TDT, CS616 and 5TE sensors (and a logarithmic calibration for the Watermark sensors) could reduce the errors of the factory calibration of θ_v to less than 0.02 ± 0.035 m³ m⁻³. Furthermore, the performance evaluation tests confirmed that each individual sensor needed a unique calibration equation for every soil type and location in the field. In addition, the calibrated van Genuchten (1980) equation was as accurate as the calibrated logarithmic equation and can be used to convert soil water potential (kPa) to volumetric soil water content (m³ m⁻³). Finally, analysis of the θ_v field data indicated that the CS616, 5TE and Watermark sensor readings were influenced by diurnal fluctuations in soil temperature, while the TDT was not influenced. Therefore, it is recommended that the soil temperature be considered in the calibration process of the CS616, 5TE, and Watermark sensors. Further research will be aimed towards determining the need of sensor calibration for every agricultural season.     

Application efficiency of micro-sprinkler irrigation of almond trees

Micro-sprinklers are becoming a preferred irrigation method for water application in orchards. However, there is relatively little data available to support a particular irrigation scheduling method. The objective of this study is to quantify the components of the water balance of an almond tree under micro-sprinkler irrigation. For that purpose, an experimental plot around an almond tree with an area of 2.0 m X 2.0 m without vegetation, representing about one quarter of the wetted area of the micro-sprinkler was instrumented with neutron access tubes, tensiometers and catch cans. Twenty-five access tubes with catch cans were distributed in a square grid of 0.5 m × 0.5 m, to a depth of 0.9 m. Eight pairs of tensiometers were installed at depths of 0.825 and 0.975 m within the experimental plot. During a 7-day period in August, 1995 the plot was sprinkler-irrigated on three days, and water application rates and uniformity coefficients were calculated for each irrigation event. Neutron probe readings at 15 cm depth increments and tensiometer readings were taken 4 to 6 times daily. Results showed large evaporation losses during and immediately after the irrigations. Evaporation losses of the wetted area was estimated to be between 2 and 4 mm/irrigation event. Consequently, application efficiencies were only 73–79%, the wetting of the root zone was limited to the 0–30 cm depth interval only, the soil profile was depleted of soil water, and daily crop coefficient values at days between irrigation events were between 0.6 and 0.8. The study recommends irrigation in the evening and night hours, thereby largely eliminating the evaporation losses that occur during daytime irrigation hours.     

Wireless lysimeters for real-time online soil water monitoring

Identification of drainage water allows assessing the effectiveness of water management. Passive capillary wick-type lysimeters (PCAPs) were used to monitor water flux leached below the root zone under an irrigated cropping system. Wireless lysimeters were developed for web-based real-time online monitoring of drainage water using a distributed wireless sensor network (WSN). Twelve PCAP sensing stations were installed across the field at 90 cm below the soil surface, and each station measured the amount of drainage water using two tipping buckets mounted in the lysimeter and continually monitored soil water contents using two soil moisture sensors installed above the lysimeter. A weather station was included in the WSN to measure micrometeorological field conditions. All in-field sensory data were periodically sampled and wirelessly transmitted to a base station that was bridged to a web server for broadcasting the data on the internet. Communication signals from the in-field sensing stations to the base station were successfully interfaced using low-cost Bluetooth wireless radio communication. Field experiments resulted in high correlation between estimated and actual drainage with r ² = 0.95 and confirmed a reliable wireless communication throughout the growing season. A web-linked WSN system provided convenient remote online access to monitor drainage water flux and field conditions without the need for costly time-consuming supportive operations.     

Cautionary notes on the use of pedotransfer functions for estimating soil hydraulic properties

The performance of published pedotransfer functions was evaluated in terms of predicted soil water content, pressure heads, and drainage fluxes for a layered profile. The pedotransfer functions developed by Vereecken et al. (1989), Vereecken et al. (1990) were used to determine parameters of the soil hydraulic functions θ(h) and K(h) which were then used as input to SWATRER, a transient one-dimensional finite difference soil water model with root uptake capability. The SWATRER model was used to simulate the hydraulic response of a multi-layered soil profile under natural climatic boundary conditions for a period of one year. The simulations were repeated by replacing the indirectly estimated water retention characteristic by (1) local-scale, and (2) field-scale mean observed θ(h) relationships. Soil moisture contents and pressure heads simulated at different depths in the soil profile were compared to measured values using these three different sets of hydraulic functions. Drainage fluxes at one meter below ground surface have also been simulated using the same three sets of hydraulic functions. Results show that simulations based on indirectly estimated moisture retention characteristics (obtained from pedotransfer functions) overpredict the observed moisture contents throughout the whole soil profile, but predict the pressure heads at shallow depths reasonably good. The results also show that the predicted drainage fluxes based on estimated retention functions are about four times as high compared to the drainage fluxes simulated using measured retention curves.     

Determination of soil hydrologic properties under simulated rainfall condition

Two important soil hydrologic properties viz. soil water retention (h-θ relationship) and unsaturated hydraulic conductivity (K-θ relationship), were determined under simulated rainfall conditions. The h-θ relationship was determined using a rainfall simulator infiltrometer (RSI). The resulting h-θ relation was then used as input to the Van Genuchten's model (VGM) for determining the K-θ relationship. In order to validate the results obtained through RSI-VGM combination, the commonly adopted instantaneous profile method (IPM) was also applied to develop the K-θ relationship independently. Functional sensitivity analysis, conducted to simulate the soil water storage using the model Soil Water Actual Transpiration Rate (SWATR), showed that the simulated results obtained through RSI-VGM combination were in agreement with those from IPM.     

Artificial neural network and time series models for predicting soil salt and water content

Volumetric water content of a silt loam soil (fluvo-aquic soil) in North China Plain was measured in situ by L-520 neutron probe (made in China) at three depths in the crop rootzone during a lysimeter experiment from 2001 to 2006. The electrical conductivity of the soil water (EC_sw) was measured by salinity sensors buried in the soil during the same period at 10, 20, 45 and 70 cm depth below soil surface. These data were used to test two mathematical procedures to predict water content and soil water salinity at depths of interest: all the available data were divided into training and testing datasets, then back propagation neural networks (BPNNs) were optimized by sensitivity analysis to minimizing the performance error, and then were finally used to predict soil water and EC_sw. In order to meet with the prerequisite of autoregressive integrated moving average (ARIMA) model, firstly, original soil water content and EC_sw time series were likewise transformed to obtain stationary series. Subsequently, the transformed time series were used to conduct analysis in frequency domain to obtain the parameters of the ARIMA models for the purposes of using the ARIMA model to predict soil water content and EC_sw. Based on the statistical parameters used to assess model performance, the BPNN model performed better in predicting the average water content than the ARIMA model: coefficient of determination (R²) = 0.8987, sum of squares error (SSE) = 0.000009, and mean absolute error (MAE) = 0.000967 for BPNN as compared to R² = 0.8867, SSE = 0.000043, MAE = 0.002211 for ARIMA. The BPNN model also performed better than the ARIMA model in predicting average EC_sw of soil profile. However, the ARIMA model performed better than the BPNN models in predicting soil water content at the depth of 20 cm and EC_sw at the depth of 10 cm below soil surface. Overall, the model developed by BPNN network showed its advantage of less parameter input, nonlinearity, simple model structure and good prediction of soil EC_sw and water content, and it gave an alternative method in forecasting soil water and salt dynamics to those based on deterministic models based on Richards’ equation and Darcy's law provided climatic, cropping patterns, salinity of the irrigation water and irrigation management are very similar from one year to the next.     

Sugarcane transpiration with shallow water-table: sap flow measurements and modelling

The most common sugarcane variety in the Gharb plain of Morocco (CP 66-345 variety) was grown in a lysimeter in the laboratory. It developed during 6 months with a water-table at 0.7 m below the soil surface. The water-table was then successively maintained with a Mariotte bottle at 0.45, 0.2 and 0.05 m from the soil surface for 21, 31 and 24 days, respectively. Transpiration was measured by Dynamax sap flow sensors. Soil water pressure heads were measured at six different depths; soil hydraulic properties and root density profile were also determined. No transpiration reduction was observed with soil waterlogging. Two different models were used to predict the pattern of root water uptake (RWU) with water-table at 0.45 m below the soil surface. These two models are based on a RWU function used as sink term in the Richards equation. The first model, HYDRUS-2D (Simunek et al., 1996), is based on the α-model RWU (Feddes et al., 1978a) which depends on a reduction function varying according to the soil water pressure head and on the root density. The second model, SIC (Breitkopf and Touzot, 1992) is based on the h_r-model RWU (Whisler and Millington, 1968, Feddes et al., 1974). It is proportional to the difference between soil and root pressure heads, to unsaturated hydraulic conductivity and to root density. Calculated soil water flows from pressure head measurements are compared to predicted pressure heads by the two models. These predictions compare well with the measured values and show that sugarcane roots mainly absorbed water in the water-table. However, while goods predictions were obtained using the actual root density profile with the h_r-model, it was necessary to modify this profile to obtain proper results using the α-model.     

Modelling soil water and salt dynamics under pulsed and continuous surface drip irrigation of almond and implications of system design

The HYDRUS-2D model was experimentally verified for water and salinity distribution during the profile establishment stage (33?days) of almond under pulsed and continuous drip irrigation. The model simulated values of water content obtained at different lateral distances (0, 20, 40, 60, 100?cm) from a dripper at 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140 and 160?cm soil depths at different times (5, 12, 19, 26 and 33?days of profile establishment) were compared with neutron probe measured values under both irrigation scenarios. The model closely predicted water content distribution at all distances, times and soil depths as RMSE values ranged between 0.017 and 0.049. The measured mean soil water salinity (ECsw) at 25?cm from the dripper at 30, 60, 90 and 150?cm soil depth also matched well with the predicted values. A correlation of 0.97 in pulsed and 0.98 in continuous drip systems with measured values indicated the model closely predicted total salts in the root zone. Thus, HYDRUS-2D successfully simulated the change in soil water content and soil water salinity in both the wetting pattern and in the flow domain. The initial mean ECsw below the dripper in pulsed (5.25?dSm^?1) and continuous (6.07?dSm^?1) irrigations decreased to 1.31 and 1.36?dSm^?1, respectively, showing a respective 75.1 and 77.6% decrease in the initial salinity. The power function [y?=?ax ^?b ] best described the mathematical relationship between salt removal from the soil profile as a function of irrigation time under both irrigation scenarios. Contrary to other studies, higher leaching fraction (6.4–43.1%) was recorded in pulsed than continuous (1.1–35.1%) irrigation with the same amount of applied water which was brought about by the variation in initial soil water content and time of irrigation application. It was pertinent to note that a small (0.012) increase in mean antecedent water content (θ _i ) brought about 8.25–9.06% increase in the leaching fraction during the profile establishment irrespective of the emitter geometry, discharge rate, and irrigation scenario. Under similar θ _i , water applied at a higher discharge rate (3.876?Lh^?1) has resulted in slightly higher leaching fraction than at a low discharge rate (1.91?Lh^?1) under pulsing only owing to the variation in time of irrigation application. The influence of pulsing on soil water content, salinity distribution, and drainage flux vanished completely when irrigation was applied daily on the basis of crop evapotranspiration (ETc) with a suitable leaching fraction. Therefore, antecedent soil water content and scheduling or duration of water application play a significant role in the design of drip irrigation systems for light textured soils. These factors are the major driving force to move water and solutes within the soil profile and may influence the off-site impacts such as drainage flux and pollution of the groundwater.     

Development and evaluation of integrated water and nitrogen model for maize

Maize (Zea mays L.) is an important food crop for irrigated regions in the world. Its growth and production may be estimated by different crop models in which various relationships between growth and environmental parameters are used. For simulation of maize growth and grain yield, a simulation model was developed (Maize Simulation Model, MSM). Dynamic flow of water, nitrogen (N) movement, and heat flow through the soil were simulated in unsteady state conditions by numerical analysis in soil depth of 0–1.8 m. Hourly potential evapotranspiration [ET_p(t)] for maize field was estimated directly by Penman–Monteith method. Hourly potential evaporation [E_p(t)] was estimated based on ET_p(t) and canopy shadow projection. Actual evaporation of soil surface was estimated based on its potential value, relative humidity of air, water pressure head and temperature at soil surface layer. Actual transpiration (T_a(t)) was estimated based on soil water content and root distribution at each soil layer. Hourly N uptake by plant was simulated by N mass flow and diffusion processes. Hourly top dry matter production (HDMA^j + 1, where j is number of hours after planting) was estimated by hourly corrected intercepted radiation (RSLT^j + 1) by plant leaves [determined from leaf area index (LAI^j + 1)] with air temperature, the maximum and minimum plant top N concentration and the amounts of nitrogen uptake. The value of LAI^j + 1 at each hour was estimated by the accumulated top dry matter production at previous hour using an empirical equation. Maize grain yield was estimated by a relationship between harvest index and seasonal plant top dry matter production. The model was calibrated using data obtained under field conditions by a line source sprinkler irrigation. When the values of water and nitrogen application were optimum, grain yield (moisture content of 15.5%) was 16.2 Mg ha⁻¹. Model was validated using two independent experimental data obtained from other experiments in the Badjgah (Fars province). The experimental results validated the proposed simulation model fairly well.     

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.     

Water use and distribution profile under pulse and oilseed crops in semiarid northern high latitude areas

Oilseed and pulse crops have been increasingly used to replace conventional summer fallow and diversify cropping systems in northern high latitude areas. The knowledge of water use (WU) and its distribution profile in the soil is essential for optimizing cropping systems aimed at improving water use efficiency (WUE). This study characterized water use and distribution profile for pulse and oilseed crops compared to spring wheat (Triticum aestivum L.) in a semiarid environment. Three oilseeds [canola (Brassica napus L.), mustard (Brassica juncea L.) and flax (Linum usitatissimum L.)], three pulses [chickpea (Cicer arietinum L.), dry pea (Pisum sativum L.) and lentil (Lens culinaris Medik.)], and spring wheat were seeded in removable 100 cm deep × 15 cm diameter lysimeters placed in an Aridic Haploboroll soil, in southwest Saskatchewan in 2006 and 2007. Crops were studied under rainfed and irrigated conditions where lysimeters were removed and sampled for plant biomass and WU at various soil depths. Wheat yields were greater than pulse crop yields which were greater than oilseed yields, and WUE averaged 4.08 kg ha⁻¹ mm⁻¹ for pulse crops, 3.64 kg ha⁻¹ mm⁻¹ for oilseeds, and ranged between 5.5 and 7.0 kg ha⁻¹ mm⁻¹ for wheat. Wheat used water faster than pulse and oilseed crops with crop growth. Pulse crops extracted water mostly from the upper 60 cm soil depths, and left more water unused in the profile at maturity compared to oilseeds or wheat. Among the three pulses, lentil used the least amount of water and appeared to have a shallower rooting depth than chickpea and dry pea. Soil WU and distribution profile under canola and mustard were generally similar; both using more water than flax. Differences in WU and distribution profile were similar for crops grown under rainfall and irrigation conditions. A deep rooting crop grown after pulses may receive more benefits from water conservation in the soil profile than when grown after oilseed or wheat. Alternating pulse crops with oilseeds or wheat in a well-planned crop sequence may improve WUE for the entire cropping systems in semiarid environments.