Feature article

Predicting soil water balance

Relationship between measured soil moisture content at Winchmore,Canterbury.

The daily time series of the observed and modelled soil moisture.

Knowing how much water is available in the soil is a key aspect of managing farm livestock feed budgets and pasture irrigation. Measurements of water content in the pasture root zone are essential to determine the water-holding characteristics of different soils. But once a reasonable number of measurements have been made, it becomes possible to model the ongoing soil water status.

The upper figure shows the relationship between measured soil moisture content at Winchmore, Canterbury, using an aquaflex sensor tape, and modelled soil moisture deficit using rainfall and evapotranspiration. The data are daily values from July 2005 to June 2006.

The figure shows that the soil moisture model explains more than 90% of the variation in the measured soil moisture. The relationship indicates that field capacity, when the modelled deficit is zero, occurs at about 34% moisture by volume. In the driest conditions, at just under 10% moisture by volume, the modelled deficit is about 130 mm. This tells us that, for this particular soil, the depth of soil water for each percent of moisture by volume is about 5.4 mm. This figure is important for irrigation planning, as it enables the farmer to calculate both the number of ‘growing days’ left before some critical deficit is reached if there is no rainfall, and the volume of water that is required in an irrigation application to lift volumetric water content by a specific amount.

The lower figure shows the daily time series of the observed and modelled soil moisture. There are some discrepancies in water content, for example in mid October and mid November where the aquaflex measurements (blue curve) show a disproportionately large response to rainfall, and these may indicate measurement errors. Apart from that, the relationship between the two datasets is reasonably consistent (although it should be noted that only one year of data has been used in this example).

These data at Winchmore are shown for a non-irrigated site. The model can also be used to monitor trigger point soil deficits, track irrigation applications, and anticipate irrigation requirements well in advance by using forecast weather conditions. Fifteen day rainfall forecasts that could be adapted for use in water balance models are available through ClimateExplorer climate-explorer.niwa.co.nz.