22/04/23 1Brainstorm session on RS applications in hydrology

Brainstorm session onremote sensing applications in hydrology

Jef Dams

22/04/23 2Brainstorm session on RS applications in hydrology Pag.

Table of content

– Remote Sensing principles– Properties of RS measurements– Remote Sensing in hydrology

22/04/23 3Brainstorm session on RS applications in hydrology Pag.

Remote Sensing principlesRemote sensing (RS) is the acquisition of information of an object or phenomenon, by the use of a device that is not in physical or intimate contact with the object

Sun and Earth electromagnetic spectrum

22/04/23 4Brainstorm session on RS applications in hydrology Pag.

Remote Sensing principles

"Atmospheric window" is the name of wavelengths where the atmosphere is "translucent" and where emission and reflection pass through almost unhindered. At other wavelengths the radiation is absorbed by various greenhouse gases

22/04/23 5Brainstorm session on RS applications in hydrology Pag.

Remote Sensing principles

‘spectral signature’

A

B(A/B)

22/04/23 6Brainstorm session on RS applications in hydrology Pag.

Table of content

– Remote Sensing principles– Properties of RS measurements– RS possibilities in hydrology

22/04/23 7Brainstorm session on RS applications in hydrology Pag.

Properties of RS measurementsSENSORS

PASSIVE SENSORS (Detect the reflected or emitted electromagnetic radiation from

natural sources.)

ACTIVE SENSORS (Detect reflected responses from objects that are irradiated from artificially-generated energy sources such as radar.)

e.g. ETM+ / MODIS / ASTER /... e.g. LIDAR / NEXRAD

22/04/23 8Brainstorm session on RS applications in hydrology Pag.

Properties of RS measurements

The vehicles or carriers for remote sensors are called the platforms.

Typical platforms are satellites and aircraft, but they can also include radio controlled aeroplanes, balloons kits for low altitude remote sensing, as well as ladder trucks or 'cherry pickers' for ground investigations.

The key factor for the selection of a platform is the altitude that determines the ground resolution and which is also dependent on the instantaneous field of view (IFOV) of the sensor on board the platform.

PLATFORM

e.g. Landsat 5,7 / TERRA / IKONOS

22/04/23 9Brainstorm session on RS applications in hydrology Pag.

Properties of RS measurements

IKONOSASTERLandsat

185 km60 km11 km

MODIS: 2330 km

- Orbit - Swath width

22/04/23 10Brainstorm session on RS applications in hydrology Pag.

Properties of RS measurements

The spectral resolution is the ability of a sensing system to resolve or differentiate electromagnetic radiations of different frequencies.The more sensitive a sensor is for small spectral differences (small wavelength intervals) the higher is its spectral resolution. Spectral resolution depends on the setting of the optical filter which splits the incoming electromagnetic radiation in smaller spectral bands.

- Panchromatic images- Multispectral images- Hyperspectral images

SPECTRAL RESOLUTION

22/04/23 11Brainstorm session on RS applications in hydrology Pag.

Properties of RS measurementsRADIOMETRIC RESOLUTION Radiometric Resolution: is determined by the number of discrete levels into which signals may be divided. e.g. 1 byte

In remote sensing, the images’ resolutions are expressed by the size of the area covered by a pixel. Each pixel in an image corresponds to a patch on the Earth’s surface. We thus talk about ‘ground resolution’.

GROUND RESOLUTION

Satellite Sensor Ground Resolution

Landsat MSS 80mLandsat Thematic Mapper 30m

SPOT XS (Multispectral) 20mSPOT Panchromatic 10mIkonos Multispectral 4mIkonos Panchromatic 1m

22/04/23 12Brainstorm session on RS applications in hydrology Pag.

Properties of RS measurements

  The temporal resolution is related ot the repetitive coverage of the ground by the remote-sensing system. For example, the temporal resolution of Landsat 4/5 is sixteen days.

TEMPORAL RESOLUTION

Orbit Swath-width

22/04/23 13Brainstorm session on RS applications in hydrology Pag.

Properties of RS measurements

spectral +temporalresolution

spatialresolution +cost / area

e.g. MODIS e.g. Quickbirde.g. Landsat

22/04/23 14Brainstorm session on RS applications in hydrology Pag.

Table of content

– Remote Sensing principles– Properties of RS measurements– Remote Sensing in hydrology

22/04/23 15Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrology  Hydrological state variables

– Land Surface Temperature– Surface soil moisture– Snow cover / snow water equivalent– Surface roughness– Land cover/use (including vegetation cover) – Water quality

  Hydrometeorological fluxes– Evapotranspiration– Snowmelt runoff– Rainfall

22/04/23 16Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrologyLAND SURFACE TEMPERATURE

The observed radiance, often brightness temperature, in the thermal spectrum is influenced by the surface temperature (which we are looking for), the atmospheric loss of intensity and the emissivity.

The emissivity is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature.

Methods to obtain LST:- ‘split window’ techniques- sensor specific methods based on the empirical relations between the range of emissivities and the minimum value from a set of multi-channel observations.

Used for:- evapotranspiration estimation- moisture- discovering bare soils

22/04/23 17Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrologySURFACE SOIL MOISTURE

At microwave frequencies (λ > 5 cm) the most striking feature of the emission form the earth’s surface is the large contrast between water and land. This is due to the high dielectric constants of water (around 80) while that of dry soils is smaller then 5.

The basic conclusion of a large number of research since the early 70’s is that it is possible to determine the moisture content of the surface layer of the soil about a ¼ of a wavelength thick (about 0-5 cm).

Main factors influencing the accuracy:- vegetation cover- soil properties- surface roughness

Used for: - data assimilation

e.g: European RS Satellite (ERS), RADARSAT (C-band), Japanese Earth Resource Satellite (JERS) and SMOS (L-band)

22/04/23 18Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrologySNOW COVER AND WATER EQUIVALENT

Large difference in physical properties of snow and other natural surfaces: e.g. high albedo (+- 80% for new snow versus +-15% for snow free surfaces).

Satellite VNIR observations: Landsat and SPOT (30m – 14d) / NOAA-AVHRR (1km – 12h) / MODIS (250m – 1d). Optimum image frequency during depletion: once a week.

The sensitivity of the microwave radiation to a snow layer on the ground makes it possible to monitor snow cover using passive microwave remote sensing techniques to derive information on snow extent, snow depth, snow water equivalent and snow state.

Used for: - snow runoff modelling

e.g: MODIS (VNIR) and AQUA (passive microwave)

22/04/23 19Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrologyLANDSCAPE ROUGHNESS

Roughness refers to the unevenness of the earth’s surface due to natural processes (e.g. topography, erosion, vegetation) or human activities (e.g. buildings)

Three complexities:- vegetation and urban roughness- transition roughness between landscape patches- topographic roughness

Used for: - evapotranspiration estimation- estimation infiltration- estimation surface water velocity

e.g: LIDAR

22/04/23 20Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrologyLAND COVER AND VEGETATION DYNAMICS

Improved remote sensing based land cover observations can improve remote sensing parameterisation. Example: Imperviousness measurements (Landsat ETM+, ASTER, IKONOS,…)

Vegetation dynamics can be derived from RS using for example vegetation indices (e.g. NDVI). High spatial versus high temporal resolution. (Landsat ETM+, MODIS, VEGETATION)

Used for: - Interception- Runoff Routing- Evapotranspiration

22/04/23 21Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrologyWATER QUALITY

Substances in surface water can significantly change the backscattering characteristics of surface water.

Major factors affecting water quality in water bodies:- Suspended sediments- Dissolved organic matter- Thermal releases

Strong research need for understanding the effects of water quality on optical and thermal properties to be able to build physically based models.

- Algae - Chemicals- …

Satellites: SEAWIFS, EOS, MOS, IKONOS

22/04/23 22Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrologyOTHER- Subsurface soil hydraulic properties (by repetitive measurements of microwave brightness temperature)- Water level (LIDAR)- Lake extend / level- Cloud cover- Wind speed- Plant Species-…

22/04/23 23Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrology  Hydrological state variables

– Land Surface Temperature– Surface soil moisture– Snow cover / water equivalent– Surface roughness– Land cover/use (including vegetation

cover) – Water quality

  Hydrometeorological fluxes– Evapotranspiration– Snowmelt runoff– Rainfall

22/04/23 24Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrologyEVAPOTRANSPIRATION

Energy balance: HGEnR

Rn is the net radiation flux (W/m²), λE is the latent heat flux (W/m²), G is the soil heat flux (W/m²) and H is the sensible heat flux to air (W/m²)

Indirect methods: Models (SVAT – ABL)

Empirical methods

Semi-empirical methods including physical componentse.g. SEBAL / SEBS / DisALexi

22/04/23 25Brainstorm session on RS applications in hydrology Pag.

Remote Sensing in hydrologyRAINFALL

Active ground based radars:

- Doppler Radar- Dual-polarized Radar- Bistatic Radar

Z=200*R^1.6

Z: This is the reflectivity factor of the precipitate. The number of drops and the size of the drops affect this value. R: This is the target range of the precipitate.

Active and passive microwave techniques (satellite based)

- Tropical Rainfall Measuring Mission (TRMM) carrying different sensors including a passive microwave sensor (TMI) (ppt over oceans) and an active sensor: TRMM precipitation Radar (PR) (ppt over land)- Future: Global Precipitation Measurement (GPM)

22/04/23 26Brainstorm session on RS applications in hydrology Pag.

  Thank you for your attendance!

22/04/23 27Brainstorm session on RS applications in hydrology Pag.