Traditionally, environmental information such as that required for hydrological applications has been provided by optical remote sensing, however, it was often hampered by time-of-day or weather constraints. In addition, the restricted penetration of optical wavelengths into a volume, such as a vegetative canopy or soil, limited the amount of information on hydrological conditions that could be derived from an image. Because SAR is an active microwave system it can provide day and night data imaging capabilities, and the low frequencies (relative to optical systems) allow for data acquisition in fog and light rain. It is particularly well suited to hydrological applications due to the sensitivity of microwave energy to the presence of water. Radar also provides greater penetration into vegetation, soil or snowpack, thus allowing surface and subsurface information to be acquired.

Field studies for hydrological applications are able to provide hydrological information at discrete points, however, these data are limited because the phenomenon being measured (e.g., soil moisture, flood extent) is highly variable over space and time, and there are difficulties in timing field data collections with dynamic events such as floods. Radar remote sensing can provide the near real time and synoptic view necessary to map hydrological features on a regional scale, and may also provide validations or supporting information for field data collection.

When radar data is acquired for quantitative hydrological study or any other application, it is important that data is calibrated as it allows image brightness values to be more directly related to target backscatter. When radar data is acquired over a number of time periods for the purpose of monitoring change for hydrological applications, it is again important that data is calibrated. Image calibration for change detection ensures that any change in the image is a result of a change in the target and not from a change in the sensor.

The hydrological applications of radar remote sensing that are of importance to water resources management include watershed modelling, flood mapping and fresh water ice mapping. The use of radar for watershed modelling involves many activities including soil moisture estimation, mapping land cover, determining wetland conditions, and snowpack condition determination. Information acquired from these activities is input into models to help predict the hydrological characteristics of a watershed. This in turn provides an estimate of the availability of free water for such activities as hydroelectricity production and crop irrigation.

The measurement of soil moisture aids in the prediction of crop yield, plant stress and watershed runoff (Brown et al. 1993). The measurement of soil moisture by C-band radar is possible due to changes in the dielectric properties of materials produced by changes in water content.

Land cover information provided by radar is an important input to watershed modelling as land cover determines in part the amount of free water available for runoff. Stream flow predictions can thus be made to determine water availability for other uses. Land cover information is possible using radar because of the differences in radar response to variations in geometric structure and moisture content associated with different land cover types.

Wetland condition determination is imperative to watershed modelling as wetlands provide vital clues to the state and availability of hydrological resources within a watershed. Mapping wetland boundaries is possible using radar because of its sensitivity to changes in the dielectric constant at the wetland boundary.

In many areas of the world, the majority of freshwater available for consumption and irrigation results from snowpack runoff. Snow wetness, snow-water equivalent and the aerial extent of the snow cover are the most important parameters in predicting total runoff. Mapping the extent of wet snow is possible using SAR data (Rott et al. 1988) as wet snow produces a low radar return in contrast to dry snow which is essentially transparent at C-band.

Flood monitoring using SAR has proven to be very successful because of the high contrast in backscatter between land and water features at microwave frequencies. The information that can be derived from temporal radar imagery in flood monitoring includes flood extent, frequency and duration. With its all-weather imaging capability, SAR provides an excellent source of data for monitoring the spatial extent and duration of floods. Flood monitoring is also important for flood damage assessment and for issues of compensation.

Fresh water ice mapping using radar is an effective tool for the evaluation of ice conditions in rivers and lakes for flood prediction. The build up of ice may prevent the normal flow of water which produces flooded conditions. Information provided from fresh water ice mapping helps in the implementation of preventative measures and evacuation procedures. Radar is capable of fresh water ice mapping due to volume and surface scattering from ice from the ice which contrasts with the specular reflection from open water.