Using soil surface temperature to assess soil evaporation in a drip irrigated vineyard
Evaporation from the soil is an important part of the water balance of a crop, when considering water use efficiency. In this paper, a non-intensive method is tested to estimate relative soil evaporation, which is based upon a linear function of soil surface temperature change between a saturated and drying soil. The relative evaporation (RE) method of Ben-Asher et al. (1983) was calibrated using microlysimeters and thermal imaging. Soil surface temperature in a drip irrigated vineyard was then collected using infrared temperature sensors mounted on a quad bike, on several days of the 2009-2010 season. Soil surface temperature in the vineyard ranged from 4.6 °C to 65.5 °C undervine and 6.8 °C to 75.6 °C in the middle of the row. The difference between daily minima and maxima of soil surface temperature ranged from 20.2 °C to 59.7 °C in the inter-row and 13.6 °C to 36.4 °C undervine. Relative evaporation averaged 54% of evaporation from a saturated soil in the inter-row and 97% undervine. Based upon the calculation of RE, the average daily amount of soil evaporation undervine was between 0.64 mm and 1.83 mm, and between 0.69 mm and 2.52 mm inter-row. The soil evaporation undervine and inter-row both exhibited spatial variability across the vineyard, however the undervine area had less spatial variability compared to the inter-row area.
Controlled water table management as a strategy for reducing salt loads from subsurface drainage under perennial agriculture in semi-arid Australia
Recent community based actions to ensure the sustainability of irrigation and protection of associated ecosystems in the Murrumbidgee Irrigation Area (MIA) of Australia has seen the implementation of a regional Land and Water Management Plan. This aims to improve land and water management within the irrigation area and minimise downstream impacts associated with irrigation. One of the plan objectives is to decrease current salt loads generated from subsurface drainage in perennial horticulture within the area from 20 000 tonnes/year to 17 000 tonnes/year. In order to meet such objectives Controlled Water table Management (CWM) is being investigated as a possible ‘Best Management Practice’, to reduce drainage volumes and salt loads. During 2000–2002 a trial was conducted on a 15 ha subsurface drained vineyard. This compared a traditional unmanaged subsurface drainage system with a controlled drainage system utilizing weirs to maintain water tables and changes in irrigation scheduling to maximize the potential crop use of a shallow water table. Drainage volumes, salt loads and water table elevations throughout the field were monitored to investigate the effects of controlled drainage on drain flows and salt loads. Results from the experiment showed that controlled drainage significantly reduced drainage volumes and salt loads compared to unmanaged systems. However, there were marked increases in soil salinity which will need to be carefully monitored and managed.
Evaluating a multi-level subsurface drainage system for improved drainage water quality
This paper describes a multi-level drainage system, designed to improve drainage water quality. Results are presented from a field scale land reclamation experiment implemented in the Murrumbidgee Irrigation Area of New South Wales, Australia. A traditional single level drainage system and a multi-level drainage system were compared in the experiment in an irrigated field setting. The single level drainage system consisted of 1.8 m deep drains at 20 m spacing. This configuration is typical of subsurface drainage system design used in the area. The multi-level drainage system consisted of shallow closely spaced drains (3.3 m spacing at 0.75 m depth) underlain by deeper widely spaced drains (20 m spacing at 1.8 m depth). Data on drainage flows and salinity, water table regime and soil salinity were collected over a 2-year period. Comparisons of water and solute movement between the multi-level drainage system and a single level drainage system are presented. Differences in the performance of the multi-level and single level drainage systems were found in the water table regime, drain water salinity and soil salinity.
Soil Spatial Variability Effects on Irrigation Efficiency
Higher evapotranspiration rates, reduced rainfall and increased water scarcity have led to a need for improved irrigation water use efficiency. Evaporation is a significant component of the total evapotranspiration (ET) and high evaporation losses reduce the amount of available water for transpiration, resulting in reduced plant water availability and hence increased irrigation. Values for soil evaporation vary widely in the literature, from 27-65% of total ET. In order to increase transpiration relative to evaporation, a reduction in evaporative losses is needed. Spatial variability of evaporation is an important factor that needs to be taken into consideration when improving water use efficiency. Soil physical properties control evaporation by influencing both the transport of water toward the soil or root surface and soil water storage. The effect of variability in soil properties on evaporation is likely to be larger in cases with water deficiency. Cultural practices such as use of narrow row spacing, mulch and minimum tillage can reduce evaporation however, they are not always effective. The potential for major savings of water depends on the length of drying interval following irrigation or rain. There is the potential for reductions in evaporation losses of up to 60% through the use of improved management techniques, enabling more water to be used by the plant for transpiration. In order to improve water use, more research in quantifying evaporation variability at the field scale needs to be completed.
Analytical Solution for Drainflows from Bilevel Multiple-Drain Subsurface Drainage Systems
Waterlogging and soil salinisation is widespread in the semiarid, irrigated areas of the world. Subsurface drainage is a useful tool in reducing these effects on crops; however, there has been negative downstream effects of drainage in the salt loads discharged to rivers, lakes, and wetlands. Thus, subsurface drainage in semiarid, irrigated areas needs to balance the demands of providing adequate waterlogging and salinity control while minimizing salt loads. Bilevel drainage, in which shallow drains are placed between deeper drains, is a potential method to meet this required balance. This paper describes the development of an analytical solution to this design approach. A previous potential theory was extended to incorporate multiple series of shallow drains placed between two deep drains. The analytical solution was then applied using the Mathematica software to provide useful information on flow rates and flow lines with varying configurations of deep and shallow drains. The theory was then used to compare spacing and drain flow characteristics between a drainage system with only deep drains and multilevel systems that combine shallow drains with deep drains. A large number of possible configurations of shallow drains between deeper drains exist. For ease of comparison, the concept of "drainage equivalence" was developed, representing the drainage discharge per unit spacing between drains. The analytical solution for bilevel drainage situations with single and multiple shallow drains between deeper drains showed that for equivalent rates of total drainage, spacing between deep drains could be increased significantly by the use of shallow drains. It also demonstrated that flow paths and drainage rates from shallow and deep drains and the total system drainage could be altered significantly by altering the number of shallow drains. This information should be useful when considering various drainage configurations to meet the dual objectives of root zone salinity control and minimization of drainage salt loads.
Controlled drainage management to minimise salt loads
This paper introduces the concept of controlled drainage and presents results from a study investigating the potential of controlled drainage for reducing drainage volumes and salt loads in an irrigated vineyard in the Murrumbidgee Irrigation Area of South Eastern Australia. This study compared traditional unmanaged drainage systems with controlled drainage systems utilizing weirs to maintain water tables and minimise drainage volumes. The results from the field experiments indicated that controlled drainage has the potential to significantly reduce drainage volumes and salt loads compared to unmanaged subsurface drainage systems. However, careful management is needed to ensure that rootzone salinity levels are maintained at optimum levels for plant production.
Estimation of soil evaporation in an irrigated vineyard from soil surface temperature
Soil evaporation is a significant unproductive loss of water that needs to be and can be managed in irrigated systems. A method is used to estimate soil evaporation based upon soil surface temperature change between a saturated and drying soil. The relative evaporation (RE) method of Ben-Asher et al. (1983) was deployed. Soil surface temperature in a drip irrigated vineyard was collected using infra-red temperature sensors. Average daily soil evaporation under-vine was between 0.6mm and 1.8mm and between 0.7mm and 2.5mm for the inter-row. Evaporation from the soil is an important part of the water balance of a crop (Burt et al. 2005). Previous estimates vary widely, from 30-65% of evapotranspiration (Kerridge et al 2008a). The Ben-Asher et al. (1983) method allows potential soil evaporation to be estimated from the daily latent fluxes of a saturated, steady-state dry and a drying soil. By calculating a relative evaporation (RE) factor and multiplying it by an estimate of potential evaporation, determined for example by the FAO-56 procedure (Allen et al., 1998), an estimate of soil evaporation may be made. The main benefit of this method is that it allows rapid and simultaneous estimates of evaporative flux to be measured at numerous sites under study. This can then be linked with methods for spatial estimation of plant water use and stress (Hornbuckle et al., 2008b).