The impact of drainage on streamflow

The primary concern of agricultural drainage engineers is, as reflected by the contents of this Monograph, the control of water table conditions within an area of crop production. However, as concern about environmental impacts continues to grow, it is becoming increasingly important to be aware that agricultural effects may extend beyond the boundary of a field or a farm. Thus, drainage will inevitably affect the pattern of water flows from the land and into the receiving watercourses. It is these `downstream' or `offsite' impacts of farmland drainage on the timing and magnitude of flows which are considered in this Chapter. The impacts of drainage on flows may be considered at two scales: the individual field outfall, where discharge can be related directly to conditions in the field (water table height, soil type, drain spacing etc), and at the river catchment scale, where not only the distributed effects of the drained areas must be integrated, but also the role of improvements to the main (arterial) channels must be considered.