|Handbook for Agrohydrology (NRI)|
|Chapter 2: Measurement of runoff|
"Runoff" is the term usually employed to distinguish the flow of water running off the land's surface during and shortly after rainfall, from the longer term flow of groundwater to rivers. This distinction is achieved by the analysis of flow data from perennial streams and rivers in humid climates, but in many agrohydrological and water harvesting situations, groundwater contributions are not present and all flow is runoff. This will almost certainly be the case in arid and semi-arid climates.
The collection of runoff data is very site and purpose-specific, both in terms of the kind of information that will be required and the manner in which it is best obtained. Runoff events are less frequent than rainfall events, in all climates. In areas of low rainfall, where agrohydrological and water harvesting projects are usually located, the number of runoff events may be fewer than ten per season. This compounds the unhelpful fact that problems with equipment and installations are not encountered until runoff occurs. If these problems are not rectified quickly, a large proportion of a season's data can be easily lost. Moreover, both equipment and experimental designs have to cope with a large range of runoff volumes and peak flows, so careful planning and a quick response to unexpected situations are very important in the success of collecting comprehensive, accurate information.
In planning runoff experiments it is important to have a clear idea of the object of research. For example:
1. In some regions, there may be no existing hydrological data. In this case it may be most suitable to spread project resources thinly and to collect information on as many different hydrological factors, at as many sites as possible. The replication of experiments will be limited and the development of hydrological models using these data will probably not be possible. A careful selection of off-station sites will be needed; adequate field staff and vehicles should be available and routine visiting schedules should be drawn. Site observers may be necessary and a greater proportion of automatic equipment will be needed. But a great number of varied circumstances will be documented and the data should be suitable for input into existing models. The information will be especially useful for projects that are following on and the research may provide insight into areas of hydrological behaviour not previously observed.
2. In other instances a much narrower focus may be desired. The development of hydrological models may be a priority and a large number of replicated experiments will be needed so that the data are amenable to statistical verification. It may be important to work cooperatively with existing projects, to extend the range of project activities and it may not be possible to undertake research in many environments.
Some projects will have a more practical applications bias and research may be combined with farmer participation and the implementation of farming systems. The overall objectives of any project will determine the financial commitment that is placed on the measurement of runoff, but it is assumed for the purpose of this guide that in the field of agricultural hydrology, runoff measurement is of primary importance.