Remote Sensing of the Land Surface May 2, 1996 North of Denver, CO August 16, 1995 Central Brazil

What colors do we need to observe? OceanPlantsSoils Urban

Visible and Near Infrared Remote Sensing

Red: nm Orange: nm Yellow: nm Green: nm Blue: nm Indigo: nm Violet: nm

Visible and Near IR Systems Panchromatic imaging system: single channel detector sensitive to radiation within a broad wavelength range; if visible range, then the resulting image resembles a "black-and-white" photograph taken from space. The physical quantity being measured is the apparent brightness of the targets. The spectral information or "color" of the targets is lost. – IKONOS PAN,SPOT,HRV-PAN Multispectral imaging system: multichannel detector with a few spectral bands. Each channel is sensitive to radiation within a narrow wavelength band. The resulting image is a multilayer image which contains both the brightness and spectral (color) information of the targets being observed. – LANDSAT MSS, LANDSAT TM, SPOT HRV-XS, IKONOS MS Superspectral Imaging Systems: many more spectral channels (typically >10) than a multispectral sensor. The bands have narrower bandwidths, enabling the finer spectral characteristics of the targets to be captured by the sensor. – MODIS, MERIS Hyperspectral Imaging Systems: "imaging spectrometer". it acquires images in about a hundred or more contiguous spectral bands. – Hyperion on EO1 satellite

Bands used for Land Surface Remote Sensing Band DesignationWavelength (nm)Application* Visible Blue Because water increasingly absorbs at longer wavelengths, this band the best data for mapping depth-detail of water-covered areas. It is also used for soil-vegetation discrimination, forest mapping, and distinguishing cultural features. Visible Green The blue-green region of the spectrum corresponds to the chlorophyll absorption of healthy vegetation and is useful for mapping detail such as depth or sediment in water bodies. Cultural features such as roads and buildings also show up well in this band. Visible Red Chlorophyll absorbs these wavelengths in healthy vegetation. Hence, this band is useful for distinguishing plant species, as well as soil and geologic boundaries. Near- IR μm This band is especially sensitive to varying vegetation biomass. It also emphasizes soil-crop and land-water boundaries in images. Near-IR μm This band is used for vegetation discrimination, penetrating haze, and water-land boundaries. Mid-IR μm This band is sensitive to plant water content, which is a useful measure in studies of vegetation health. It is also used to distinguish clouds, snow, and ice. Mid-IR μm This band is used for mapping geologic formations and soil boundaries. It is also responsive to plant and soil moisture content. Table 1. Visible and IR bands used for Land Surface Studies *Application synthesis adapted from Yale University Remote Sensing and GIS Research Group

PAR Action Spectrum Photosynthetically Active Radiation violet - blue - green-yellow-orange - red - near IR

Measuring Vegetation

Strong Reflection Strong Differential Absorption

Attenuation in the Visible Wavelengths (molecular/no aerosol) Grant Petty, 2004 Blue and light blue Scattered by molecules ozone 765 nm 865 nm

Blue and Light Blue Direct Beam Diffuse

PAR Action Spectrum Photosynthetically Active Radiation violet - blue - green-yellow-orange - red - near IR Blue Line: depiction of molecular scattering in the visible wavelength bands

Attenuation in the Visible Wavelengths Grant Petty, 2004

Aerosols scatter downwelling and upwelling visible radiation Haze Bloom?

Daytime Visibility Distant Dark Objects Appear Brighter “Clear” Day Hazy Day

Daytime Visibility White Sunlight Top of Atmosphere Color and Intensity Distance to the Dark Object consider scattering by aerosols

Daytime Visibility White Sunlight Top of Atmosphere Increased contribution of white light Object appears lighter with distance Longer Distance to the Dark Object

Daytime Visibility Distant Dark Objects Appear Brighter “Clear” Day Hazy Day

What the satellite sees White Sunlight Top of Atmosphere molecular and aerosol scattering 400→ 500 nm ocean water nm plants nm and near-IR atmosphere: windows in near IR M A G NIR

Atmospheric Aerosol Correction Procedure BlueGreenRedNear-IR Ln (Optical Thickness) Cloudy Cloudless-Polluted Molecular Scattering Aerosols Satellite Channels Aerosol Molecules Surface

NDVI NDVI is calculated from the visible and near- infrared light reflected by vegetation. Healthy vegetation – absorbs visible light and reflects a large portion of the near-IR light Unhealthy or sparse vegetation – reflects more visible light and less near-IR light Real vegetation is highly variable

NDVI NASA Earth Observatory (Illustration by Robert Simmon)

NOAA 11 AVHRR NOAA 7 AVHRR NOAA 9 AVHRR NOAA 14 AVHRR SeaWiFS SPOT MODIS NOAA-16 NPP NOAA 9 NOAA-17 Satellite NDVI data sources NOAA-18 C. Tucker

Terra Satellite December 1999: Terra spacecraft Moderate-resolution Imaging Spectroradiometer, or MODIS, that greatly improves scientists’ ability to measure plant growth on a global scale.MODIS, MODIS: higher spatial resolution (up to 250-meter resolution) than AVHRR

MODIS Global NDVI

Average NDVI ~40,000 images composited C. Tucker Green  NDVI  1

Marked contrasts between the dry and wet seasons Senegal

Beltsville USA winter wheat biomass C. Tucker

Remote Sensing of Soil Moisture Lecture 7

What is soil moisture? Soil moisture  water that is held in the spaces between soil particles. – Surface soil moisture is the water that is in the upper 10 cm of soil – Root zone soil moisture is the water that is available to plants, which is generally considered to be in the upper 200 cm of soil. Ratio of liquid water content to the soil in percentage of volume or weight  hysteresis: memory of previous precipitation events. Soil moisture is a key variable used to describe water and energy exchanges at the land surface/atmosphere interface

Thermal infrared techniques Microwave – Active – Passive Optical (visible/near infrared) Remote Sensing of Soil Moisture

Advantages of Microwave RS Transparent atmosphere Vegetation semitransparent Microwave measurement strongly dependent on dielectric properties of soil water Not dependent on solar illumination

Basis for Microwave Remote Sensing of Soil Moisture Basis for microwave remote sensing of soil moisture is contrast in dielectric constant of water (80) and dry soil (<5), causing emissivity contrast of 0.4 for water and 0.95 for dry land (Schmugge 2002) Research concludes surface layer sm can be determined to about ¼ wavelength, i.e. 0-5 cm layer using microwave λ = 21 cm Longer λ better for increased depth, less noise

Soil moisture in pasture λ = 21 cm responded Soil moisture  λ = 21 cm  Schmugge 2002

Emissivity and Soil Moisture Brightness temperature related to emissivity for 0 to 5 cm surface layer ε M is soil surface emissivity, T M is soil surface temperature (1-ε M )T sky is ~ 2K, therefore ε M ~ T B /T M If T M estimated independently, ε M can be determined Typical range for ε M is 0.9 for dry soil to 0.6 for smooth wet soil T B = ε M T M + (1-ε M )T sky Schmugge 2002

Factors affecting accuracy Vegetation cover – Most important, dense vegetation (corn, forest) can obscure soil surface – Greater effect at shorter λ Soil properties – Density and texture Surface roughness – Commonly 10 to 20% reduction in response range Density and roughness relatively constant

Radar Remote Sensing— Soil Moisture HYDROS ( – Back-up ESSP mission for global soil moisture. L-band radiometer. L-band radar. – Died mission Courtesy: Tom Jackson, USDA SGP’97 Radar Pol: VV, HH & HV Res – 3 and 10 km Radiometer Pol: H, V Res =40 km, dT= 0.64º K