15.1 Introduction
Soil moisture, evapotranspiration (ET; a combination of evaporation and transpiration in mm), and agricultural drought are so closely interlinked phenomena that
deficiency in soil moisture causes reduction in ET, which eventually leads to
vegetative stress and agricultural drought.
While surface soil moisture, surface available energy, and atmospheric resistance
to water vapor transport drive evaporation rate from soil, transpiration is controlled
by a combination of factors including moisture availability in the plant’s root zone,
available energy, and atmospheric and canopy resistances to water vapor transport
(Monteith 1965). A lack of water in the root zone leads to stomatal closure, reduced
transpiration, and photosynthesis. As photosynthesis slows down, nutrition
exchange between roots and leaves gradually diminishes, and green vegetation starts
to lose its turgor and wither. As a result, agricultural drought develops after the
degradation of healthy vegetation. Dramatic crop losses and decline in crop yields
emerge especially when crops are experienced drought during the critical stage of
their growing season, thereby adversely impacting the agricultural yield and the
economy as seen in the 2011 Texas drought in the United States (US) (NielsenGammon 2012) and US drought of 2012 (Rippey 2015). According to the US
Drought Monitor (USDM) between July 10, 2012, and September 18, 2012 (The
National Drought Mitigation Center 2017), approximately 80% of the
U.S. experienced drought with varying severities from abnormally dry to extreme
drought. As a result, the US drought of 2012 resulted in poor hay/pasture conditions
inadequate to support the demands of the livestock industry, increased crop abandonment (e.g., cotton in Texas, United States), and a significant drop in corn,
soybean, and winter wheat yields in Midwest United States (Rippey 2015).
Monitoring of soil moisture, ET, and vegetation condition is indispensable to
identify terrestrial drought conditions to take proactive measures and mitigate its
negative consequences. Although conventional methods such as point-based measurements of soil moisture, latent heat (LE; energy equivalent of ET in Wm
À2 ),
precipitation and temperature records measured at the ground stations (e.g., soil
moisture networks, meteorological stations, and eddy-covariance flux towers) provide accurate discreet observations, they are unable to accurately capture conditions
in places between the ground stations (Brown et al. 2008). Therefore, techniques
based on remote sensing inputs are important to track spatially continuous estimates
of soil moisture, ET, and vegetation condition from regional to global scales, while
the point-based estimates of soil moisture, ET, precipitation, and temperature can be
utilized to assist in validating satellite-based estimates of soil moisture, ET, and
agricultural drought. In this regard, several polar-orbiting satellite missions have
been dedicated to generating soil moisture, precipitation, and vegetation condition
products from space such as the Soil Moisture Ocean Salinity (SMOS), Advanced
Microwave Scanning Radiometer (AMSR2), and Soil Moisture Active Passive
(SMAP) instruments for soil moisture retrieval, the Tropical Rainfall Measuring
Mission (TRMM) and Global Precipitation Measurement (GPM) for precipitation,
300
A. L. Yagci and M. T. Yilmaz
Soil moisture, evapotranspiration (ET; a combination of evaporation and transpiration in mm), and agricultural drought are so closely interlinked phenomena that
deficiency in soil moisture causes reduction in ET, which eventually leads to
vegetative stress and agricultural drought.
While surface soil moisture, surface available energy, and atmospheric resistance
to water vapor transport drive evaporation rate from soil, transpiration is controlled
by a combination of factors including moisture availability in the plant’s root zone,
available energy, and atmospheric and canopy resistances to water vapor transport
(Monteith 1965). A lack of water in the root zone leads to stomatal closure, reduced
transpiration, and photosynthesis. As photosynthesis slows down, nutrition
exchange between roots and leaves gradually diminishes, and green vegetation starts
to lose its turgor and wither. As a result, agricultural drought develops after the
degradation of healthy vegetation. Dramatic crop losses and decline in crop yields
emerge especially when crops are experienced drought during the critical stage of
their growing season, thereby adversely impacting the agricultural yield and the
economy as seen in the 2011 Texas drought in the United States (US) (NielsenGammon 2012) and US drought of 2012 (Rippey 2015). According to the US
Drought Monitor (USDM) between July 10, 2012, and September 18, 2012 (The
National Drought Mitigation Center 2017), approximately 80% of the
U.S. experienced drought with varying severities from abnormally dry to extreme
drought. As a result, the US drought of 2012 resulted in poor hay/pasture conditions
inadequate to support the demands of the livestock industry, increased crop abandonment (e.g., cotton in Texas, United States), and a significant drop in corn,
soybean, and winter wheat yields in Midwest United States (Rippey 2015).
Monitoring of soil moisture, ET, and vegetation condition is indispensable to
identify terrestrial drought conditions to take proactive measures and mitigate its
negative consequences. Although conventional methods such as point-based measurements of soil moisture, latent heat (LE; energy equivalent of ET in Wm
À2 ),
precipitation and temperature records measured at the ground stations (e.g., soil
moisture networks, meteorological stations, and eddy-covariance flux towers) provide accurate discreet observations, they are unable to accurately capture conditions
in places between the ground stations (Brown et al. 2008). Therefore, techniques
based on remote sensing inputs are important to track spatially continuous estimates
of soil moisture, ET, and vegetation condition from regional to global scales, while
the point-based estimates of soil moisture, ET, precipitation, and temperature can be
utilized to assist in validating satellite-based estimates of soil moisture, ET, and
agricultural drought. In this regard, several polar-orbiting satellite missions have
been dedicated to generating soil moisture, precipitation, and vegetation condition
products from space such as the Soil Moisture Ocean Salinity (SMOS), Advanced
Microwave Scanning Radiometer (AMSR2), and Soil Moisture Active Passive
(SMAP) instruments for soil moisture retrieval, the Tropical Rainfall Measuring
Mission (TRMM) and Global Precipitation Measurement (GPM) for precipitation,
300
A. L. Yagci and M. T. Yilmaz