Rain, soil moisture and urbanisation

The amount of runoff from a catchment depends on the amount of rain, but it also the case that the amount of rain depends on the catchment.  Catchments influence the amount of rain in different ways in rural, irrigated and urban areas.

In rural catchments, some recent work has shown a relationship between summer afternoon rain and soil moisture.  When there is a lot of soil moisture around i.e. in wet summers, rainfall is more frequent – that is to be expected – but the rain is most likely to fall over areas of dry soil.  This is because sun shining on these dry areas produces the warmest air that rises further and faster.  Moisture in the air then condenses to form rain.  There is a nice summary of the research here and in this letter to Nature.

Now, new research has suggested rainfall is less likely over irrigated, compared to non-irrigated areas.  Air is more stable over irrigated regions because they are cooled by evaporating water.  The air doesn’t rise as far, or as fast on a warm afternoon so rain is less frequent.  This causes more dry days and reduced rainfall totals.  A summary of the research is available here.

There is likely a similar effect in urban areas.  In summer, cities tend to be warmer than surrounding non-urban land which can lead to more frequent and more intense afternoon rainfall (see some great research on rainfall over Houston).  However the total amount of rain over an urban area can decrease because the effect of development can lead to less water in the landscape.  This is consistent with cities being dry and hot in summer.   For example research in the large urban area of Beijing showed fewer but more intense summer rainfall events.

Increased rainfall intensity isn’t the only factor contributing to changes in flooding regime caused by urbanisation.  The flood regime of a stream can change under the influence of three factors:

  1. Atmospheric processes e.g. increased rainfall intensity, as mentioned above.
  2. Catchment processes e.g. an increase in impermeable areas or other landuse change.
  3. Alteration to a stream e.g. improving hydraulic efficiency – piping and lining, decreasing floodplain storage, or extending the drainage network – the underground drainage system that reaches to the furthest corners of a developed catchment.

In urban areas, all three factors are at work, so floods become larger and more frequent following urbanisation.  Plus there are are a lot more people and assets that are vulnerable to flooding.

ThreeHeadedDog

The three-headed dog of urbanisation impacts on flooding

Further reading

Burian, S. J. and Shepherd, J. M. (2005) Effect of urbanization on diurnal rainfall pattern in Houston.  Hydrological Processes 19(5):1089-1103 (link).

Konrad, C. P. (2014) Effects of urban development on floods.  U. S. Geological Survey. Fact Sheet 076-03 (link) (pdf).

Selman, C. and Mishra, V. (2016) The sensitivity of southern US climate to varying irrigation vigor.  Journal of Geophysical Research Atmos.  DOI: 10.1002/2016JD025002 (link).

Taylor, C. M., de Jeu, R. A. M., Guichard, F., Harris, P. P. Dorigo, W. A. (2012) Afternoon rain more likely over drier soils.  Nature doi:10.1038/nature11377 (link).

Viglione, A., Merz, B. Dung, N. V., Parajka, J., Nester, T. and Bloschl, G. (2016) Attribution of regional flood changes based on scaling fingerprints.  Water Resources Research DOI: 10.1002/2016WR019036 (link).

 

 

 

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