What is the Topographic Wetness Index (TWI) layer?

What is the Topographic Wetness Index Layer and how do I use it?

The Topographic Wetness Index (TWI) is a model that provides information on the degree of soil wetness in a particular area based on the topography of the land. TWI is calculated using the slope and flow accumulation of the terrain and can be used to identify areas of the landscape with high or low soil moisture content.

FarmLab calculates this using data both slope and elevation data provided by the Shuttle Radar Topography Mission (SRTM), was a joint project between NASA, the National Geospatial-Intelligence Agency (NGA), and the German Aerospace Center (DLR). The project used a specially modified radar instrument on the space shuttle Endeavour to collect elevation data for most of the Earth's land surfaces, from 56 degrees south to 60 degrees north latitude.

In agriculture, TWI can be useful for a number of reasons:

  1. Drainage management: TWI can help farmers identify areas of their fields that are likely to be prone to waterlogging, which can lead to reduced crop yields or even crop failure. By understanding the wetness of the soil in different parts of their fields, farmers can make decisions about drainage management practices such as installing drainage tiles or creating channels to direct water away from sensitive areas.

  2. Irrigation management: TWI can also be used to identify areas of the field that are more or less likely to require irrigation. By focusing irrigation on areas with low TWI values, farmers can optimise water use efficiency and reduce the risk of overwatering.

  3. Crop selection: TWI can also help farmers to choose the most appropriate crops for different parts of their fields. Crops that are tolerant of wet soils may be more suitable for areas with high TWI values, while crops that require well-drained soils may be better suited to areas with low TWI values.

  4. Conservation management: TWI can also be used to identify areas of the landscape that are prone to erosion or sedimentation. By focusing conservation practices such as cover crops or reduced tillage on these areas, farmers can help to maintain soil fertility and reduce nutrient runoff into nearby waterways.

In summary, TWI can provide valuable information for farmers to optimise drainage, irrigation, crop selection, and conservation management practices, which can help to improve agricultural productivity and sustainability.