Recent advances in Recent advances in remote sensing in remote sensing in

hydrology hydrology

IntroductionIntroduction Remote sensing has held a great deal of promise for hydrology, Remote sensing has held a great deal of promise for hydrology,

mainly because of the potential to observe areas and entire river mainly because of the potential to observe areas and entire river basins rather than merely points.basins rather than merely points.

Most of the advances in using remote sensing for hydrology have Most of the advances in using remote sensing for hydrology have come from new areas of hydrologic analysis; areas where existing come from new areas of hydrologic analysis; areas where existing methods were unsatisfactory or limiting and areas where sufficient methods were unsatisfactory or limiting and areas where sufficient data were sparse or nonexistent. data were sparse or nonexistent.

In this reviewIn this review, the various applications of remote sensing to , the various applications of remote sensing to hydrology are treated as they are used to measure the different hydrology are treated as they are used to measure the different hydrologic variables or processes related to the water and energy hydrologic variables or processes related to the water and energy cycle; i.e. cycle; i.e. precipitationprecipitation, , snowsnow, , evaporationevaporation, etc. Each of these , etc. Each of these hydrologic variables or processes is discussed individually with the hydrologic variables or processes is discussed individually with the emphasis on how remote sensing is being used, and not on the emphasis on how remote sensing is being used, and not on the technology as far as sensor details and specific instruments is technology as far as sensor details and specific instruments is concernedconcerned

Remote SensingRemote Sensing

Remote sensing uses measurements of the electromagnetic Remote sensing uses measurements of the electromagnetic spectrum to characterize the landscape, or infer properties spectrum to characterize the landscape, or infer properties

of it, or in some cases, actually measure hydrologic state variables.of it, or in some cases, actually measure hydrologic state variables.

Different sensors can provide unique information about properties of Different sensors can provide unique information about properties of the surface or shallow layers of the Earth. the surface or shallow layers of the Earth.

For exampleFor example, , Measurements of the reflected solar radiation give Measurements of the reflected solar radiation give information on albedo, information on albedo, thermal sensorsthermal sensors measure measure surface temperature, and surface temperature, and microwave sensorsmicrowave sensors measure the dielectric properties and hence, the measure the dielectric properties and hence, the moisture content, of surface soil or of snowmoisture content, of surface soil or of snow . .

PrecipitationPrecipitation Hydrologists have increasingly turned to remote Hydrologists have increasingly turned to remote

sensing as a possible means for quantifying the sensing as a possible means for quantifying the precipitation input, especially in areas where precipitation input, especially in areas where there are few surface gauges.there are few surface gauges.

Direct measurement of rainfall from satellites for Direct measurement of rainfall from satellites for operational purposes has not been generally operational purposes has not been generally feasible. feasible.

Satellites provide frequent observations, the Satellites provide frequent observations, the characteristics of potentially precipitating clouds characteristics of potentially precipitating clouds and the rates of changes in cloud area and and the rates of changes in cloud area and shape can be observed. shape can be observed.

Precipitation (Contd..)Precipitation (Contd..)

The availability of meteorological and Landsat satellite data has The availability of meteorological and Landsat satellite data has produced a number of techniques for inferring precipitation from the produced a number of techniques for inferring precipitation from the visible and/or infrared (VIS/IR) imagery of cloudsvisible and/or infrared (VIS/IR) imagery of clouds..

The The GOES Precipitation IndexGOES Precipitation Index [ [ Arkin, Arkin, 1979], derived from 1979], derived from thresholding the infrared brightness temperature of cloud tops has thresholding the infrared brightness temperature of cloud tops has been used to study the distribution of tropical rainfall.been used to study the distribution of tropical rainfall.

The university of Bristol has led the development of a The university of Bristol has led the development of a cloud indexingcloud indexing approach.approach.

A A life-historylife-history approach, developed by approach, developed by Scofield and Oliver,Scofield and Oliver, [1977] [1977] considers the rates of change in individual convective clouds or considers the rates of change in individual convective clouds or clusters of convective clouds. This approach is the basis of a flash clusters of convective clouds. This approach is the basis of a flash flood system that assimilates GOES data with ground based and flood system that assimilates GOES data with ground based and atmospheric data to forecast precipitation amounts for use in a flood atmospheric data to forecast precipitation amounts for use in a flood forecast model [forecast model [ Clark and Morris, Clark and Morris, 1986]. 1986].

Snow HydrologySnow Hydrology Snow is a form of precipitation; Snow is a form of precipitation;

Nearly all regions of the electromagnetic spectrum Nearly all regions of the electromagnetic spectrum provide useful information about the snow pack.provide useful information about the snow pack.

The water equivalent of snow can be measured from low The water equivalent of snow can be measured from low elevation aircraft carrying sensitive gamma radiation elevation aircraft carrying sensitive gamma radiation detectors detectors [ Carroll and Vadnais, 1980][ Carroll and Vadnais, 1980]

Snow can readily be identified and mapped with the Snow can readily be identified and mapped with the visible bands of satellite imagery because of its high visible bands of satellite imagery because of its high reflectance in comparison to non-snow areas.reflectance in comparison to non-snow areas.

Snow Hydrology (Contd..)Snow Hydrology (Contd..)

Microwave remote sensing offers great promise for future Microwave remote sensing offers great promise for future applications to snow hydrology. applications to snow hydrology.

Active microwave remote sensing also has the potential to provide Active microwave remote sensing also has the potential to provide important information about the snow pack and at very high important information about the snow pack and at very high resolution with Synthetic Aperture Radar (SAR), [resolution with Synthetic Aperture Radar (SAR), [ Stiles et al, Stiles et al, 1981, 1981, and and Rott,Rott, 1986 1986]. ].

The Snowmelt Runoff Model (SRM) [The Snowmelt Runoff Model (SRM) [ Martinec et al, Martinec et al, 1983 1983] was ] was specifically developed for using remote sensing of snow cover by specifically developed for using remote sensing of snow cover by elevation zone as the primary input variable. elevation zone as the primary input variable.

Runoff and Hydrologic Modeling Runoff and Hydrologic Modeling

Runoff cannot be directly measured by remote sensing techniques. Runoff cannot be directly measured by remote sensing techniques. (1)(1) determining watershed geometry, drainage network, and other determining watershed geometry, drainage network, and other map-type information for distributed hydrologic models and for map-type information for distributed hydrologic models and for empirical flood peak, annual runoff or low flow equations; and empirical flood peak, annual runoff or low flow equations; and

(2)(2) providing input data such as soil moisture or delineated land use providing input data such as soil moisture or delineated land use classes that are used to define runoff coefficients. classes that are used to define runoff coefficients.

Drainage basin areas and the stream network are easily obtained Drainage basin areas and the stream network are easily obtained from good imagery, even in remote regions. from good imagery, even in remote regions.

Empirical flood formulae are useful for making quick estimates of Empirical flood formulae are useful for making quick estimates of peak flow when there is very little other information available. peak flow when there is very little other information available.

Landsat data have been used to improve empirical regression Landsat data have been used to improve empirical regression equations of various runoff characteristics. equations of various runoff characteristics.