Assessing low flow impact of plantation forestry on mini-catchment spot gauging

 SR 97(09)F

 April 1998

Executive summary

This report details a programme of streamflow measurement undertaken over the summer of 1996, aimed at improving the understanding of plantation forestry on summer low flows. The study employed a specifically-chosen methodology of making periodic flow measurements (usually by volumetric gauging) across two networks of 13 and 44 mini-catchments respectively, as a means of assessing low flow behaviour in small areas corresponding to simple land use situations and with only modest monitoring effort.

Streams were monitored in two contrasting areas, both supporting some plantation forestry, namely the Trossachs, representative of wet, upland conditions, and Angus and Kincardineshire, representative of the drier lowlands. Continuous flow, ephemeral and continuously dry behaviour was found in the monitoring network, with varying degrees of responsiveness to rainfall.

Overall, there was a strong suggestion that in many streams, summer flows from forested catchments were higher than in non-forested catchments - a surprising result thought to result from surface drains causing enhanced runoff. This was also apparent through many forested streams in the Trossachs producing a more sustained response to rainfall (up to 25 days) than grassland streams. The effects of catchment geology were apparent through the marked variability in flow between adjacent streams.

Based on the field data collected, the report suggests that decisions regarding future plans for afforestation should take into account the possible enhancement of summer low flows by surface drainage, and also the localised factors controlling surface runoff, such that individual small streams may need to be identified for protection from any possible disturbance. It should be borne in mind, however, that the study explored streamflow behaviour in only two broad areas and over only one summer, and that they may not necessarily represent conditions elsewhere.

The results obtained are found to be instructive, and the methodological progress made in establishing networks of many contrasting streams for periodic visits, and in gauging by volumetric methods, is considered to be suitable encouragement for considering plans for further research employing these methods.

Copies of the report are available from FWR, price 15.00, less 20% to FWR Members