Soil water content varies spatially and temporally on landscapes due to various processes including water loss by evapotranspiration (ET). When rainfall is not
sufficient to meet water demand caused by ET losses from crop fields, irrigation becomes necessary to maintain sustainable crop productivity. More efficient use of
water from rainfall and irrigation is required at the field scale through improved irrigation scheduling to improve our understanding of crop response to water deficit
Most of the previous research in spatially variable crop production has
focussed on variable rate fertilizer and chemical application. The research reported here is aimed at extending precision farming concepts to precision water
management to ensure water is applied in the right place, in the right amount, and at the right time, to optimise production and efficiency. In order to determine the
feasibility and applicability of precision water management, experiments were undertaken to:
1. test and evaluate the performance of a load-cell-based mini-lysimeter inside a glasshouse to determine evapotranspiration losses with high precision;
2. test the prospects of thermal sensing of crop plants with a thermal infrared camera (infrared thermography) to identify the relationship between canopy temperature and soil water including the physiological basis of crop water
3. examine the spatial variability in soil water content with electrical conductivity measurements with EM38 (based on the electromagnetic induction technique).
Field and glass house experiments were conducted with cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.) crops using a self-mulching, black vertosol soil. All experiments consisted of four irrigation treatments (either based on irrigation when soil water content was depleted to a known percent of plant available water capacity (PAWC) or field capacity (FC). All experiments used a randomized
complete block design involving 3 to 5 replications of each irrigation treatment. Irrigation treatments (T50, T60, T70 and T85) in the field were designed to allow soil water depletion to 50%, 60%, 70% and 85% of the PAWC in the soil for both wheat and cotton crop. Irrigation treatments used for the glasshouse experiment included: T80 – 80% of FC, T70 – 70% of FC, T50 – 50% of FC and T40 – 40% of
FC. Frequency of irrigation in all experiments varied over time to allow the soil water deficit to develop as required for the irrigation treatments. Two field (2007-08,
2008-09) and one glasshouse (2008) experiments were conducted for cotton, while one field (2008) and a glasshouse experiment (2009) was conducted for wheat.
Measurements in all experiments included essential weather data (rainfall, relative humidity, solar radiation, and maximum and minimum air temperature), volume of irrigation and drainage (for glasshouse experiments only), soil water
content, yield and biomass of the crops. On selected occasions, thermal images were taken with an NEC TH7800 infrared camera before and/or after irrigation both in the
field and glasshouse experiments. Canopy temperature was derived from processing of the thermal images. Leaf water potential was measured with a pressure chamber and stomatal conductance of leaves measured with a steady state porometer at the time of thermal imaging. All measurement positions in the field were recorded with a hand-held GPS. Images of wet (leaf covered with water on both sides of the leaf) and dry reference (leaf covered with petroleum jelly) leaves were taken for each irrigation treatment at the time of image acquisition of normal leaves. The
temperatures of normal, wet and dry reference leaves were used in the calculation of crop water deficit indices such as ICWSI (Improved Crop Water Stress Index) and IG
(Index relating to stomatal conductance). For the field experiments, apparent electrical conductivity (ECa) was measured on the same day as the other measurements with the EM38 equipment in both vertical and horizontal modes on
the ground as well as 0.1 m and 0.4 m above the ground. Soil temperature within 0-25 cm depth was recorded with a resistance temperature detector (RTD) probe.
Results indicated the ability of thermal imaging to consistently distinguish water deficit in crops for the most frequently irrigated treatments (i.e. T50 in the
field and T80 in the glasshouse) from the least frequently irrigated treatments (i.e. T85 treatment in the field and T40 in the glasshouse). Due to the strong dependency
of canopy temperature on water relations of leaves (leaf water potential and stomatal conductance); it was possible to ascertain the extent of soil water availability within
the root zone of crops to maintain an optimum transpiration rate in leaves of the studied crops. The relationship between crop water deficit indices (i.e. ICWSI and
IG) and soil water within the root zone were also explored. Maps of spatial variations in canopy temperature for the entire field matched well with soil water maps. Due to
the close correspondence between soil water deficit and canopy temperature, these maps are expected to be useful for precision irrigation. The trends between thermal
data and other indicators (leaf water potential, stomatal conductance and soil water) suggest that thermography is a rapid and convenient approach to detect crop water
deficit stress in the field, when soil water deficit may vary randomly due to existing variation in soil and/or landscape properties and water management.
Measurement of ECa with EM38 equipment was also found to be useful in assessing spatial distribution of soil water content and water deficit stress in crop fields. Since ECa is a complex function of several variables including temperature, absolute quantities of soil water can be predicted in crop fields with only moderate
accuracy, however wet and dry areas within the field can be easily identified. It is clearly possible to incorporate information on spatial variability in soil water
content/deficit in the field directly with ECa maps and/or with thermal imagery (as a crop property affecting transpiration) to improve the effectiveness of irrigation by
irrigating spatially variable fields at a high precision.
With the mini-lysimeter system described in this work, it is possible to measure ET losses from crops at a resolution of 0.027 mm with a time interval of approximately 10 min, which is ideal for studying spatial and temporal variability in growth and performance of irrigated crops.
An irrigation strategy is considered effective if it does not cause significant yield reduction while allowing highest possible water use efficiency to be maintained. Analysis of yield and ET data for cotton and wheat indicates that both crops should be irrigated when soil water content depletes to 60% of PAWC in the
field or 70% of FC in the glasshouse. Since soil water content and its spatial distribution can be estimated in the field with thermal imagery or EM38 measurements, irrigation can be applied to maintain soil water content above these limits throughout the field.