Runoff is fallen precipitation that moves across the land’s surface. Often runoff only begins to occur when the soil is saturated (saturation-excess runoff) but if the infiltration rate is less then the precipitation rate then runoff can occur before the soil is saturated. Termed infiltration-excess runoff, this type of runoff is common in hydrophobic soils. Runoff is most likely to end up in a watercourse, but can become soil water or percolate into an aquifer after initially running off. Runoff is influenced by both rainfall and catchment characteristics, for example, rainfall intensity, spatial distribution of rainfall, pre-storm soil conditions, topography and land use.
How to estimate runoff
Runoff can be estimated by looking at available data or by using equations or groups of equations which form models.
By comparing the rainfall total of a storm to the discharge observed in a watercourse, a rough estimate of runoff can be calculated.
Engineers have developed equations to estimate runoff.
There are multiple model that output runoff, among other variables, including AWRA-L and eWater Source. Both AWRA-L and the eWater model include a rainfall-runoff component. A rainfall-runoff model derives runoff for an area by taking into account the rainfall the area received and the amount of evaporation from the area.
Runoff in the Gilbert
As with rainfall, runoff in the Gilbert catchment is concentrated in the wet season, when 98% of yearly runoff occurs. However, runoff is highly variable between years, depending on the strength of the corresponding wet season. This can be seen when looking at the mean, compared to the maximum and minimum yearly runoff amounts. The mean annual runoff averaged over the Gilbert catchment is 140 mm. The largest catchment annual runoff was observed in 1974, totaling 1231 mm. In comparison, the smallest catchment annual runoff was observed in 1935, totaling only 4 mm. Annual median runoff is less then mean runoff, 100 mm and 140 mm respectively. This means that the distribution is positvely skewed (i.e., there are more small values). The mean is more influenced by extremes so would be more infleunced by a few extreme wet years then the median would.
Runoff in the catchment generally increases heading downstream.
Resources
- CSIRO Agricultural resource assessment for the Gilbert catchment
- CSIRO Calibration of river models for the Flinders and Gilbert catchments.
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