# Optical and Thermal Remote Sensing for Monitoring Agricultural Drought
###### Abstract  
By effectively observing the land surface and obtaining farmland conditions, satellite remote sensing has played an essential role in agricultural drought monitoring over past decades. Among all remote sensing techniques, optical and thermal remote sensing have the most extended history of being utilized in drought monitoring. The primary goal of this paper is to illustrate how optical and thermal remote sensing have been and will be applied in the monitoring, assessment, and prediction of agricultural drought. We group the methods into four categories: optical, thermal, optical and thermal, and multi-source. For each category, a concise explanation is given to show the inherent mechanisms. We pay special attention to solar-induced chlorophyll fluorescence, which has great potential in early drought detection. Finally, we look at the future directions of agricultural drought monitoring, including (1) early detection; (2) spatio-temporal resolution; (3) organic combination of multi-source data; and (4) smart prediction and assessment based on deep learning and cloud computing. agricultural drought monitoring; optical remote sensing; thermal remote sensing; drought indices; solar-induced fluorescence +
Footnote †: journal: Remote Sensing  
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Since the 1950s, with accelerated industrialization and urbanization, global warming has become undoubtedly prominent. The frequency and intensity of extreme climate events caused by global warming have increased significantly, as has the drought vulnerability in many regions all over the world [9; 10]. As a sector most constrained by climate and weather [10], agriculture is greatly impacted by weather-induced disasters, especially drought. When a long-lasting drought is not handled properly, a food crisis usually follows closely, thus causing famine and mortality. Based on the Emergency Events Database (EM-DAT) ([[https://www.emdat.be](https://www.emdat.be)]([https://www.emdat.be](https://www.emdat.be)), accessed on 20 August 2018), we have compiled the most severe drought events (considering total economic damage and human deaths) that happened after 1960 into Figure 1. We can see the great impact of drought on agriculture from the coincidence of drought events and major agricultural areas. During the growth period, crop water use mainly involves transpiration and metabolic consumption. When agricultural drought occurs, the soil water deficit leads to reduced soil evaporation and capillary edge water. Crops become unable to take in enough water via their roots to keep their water balance. As a result, leaf transpiration decreases [11]. Since soil evaporation and crop transpiration are two main contributors to farmland evapotranspiration, the decline of both causes the total evapotranspiration to drop; consequently, both soil and canopy temperature rise, which aggravates the drought condition. When the drought is severe enough, crop leaves and stalks begin to show notable physiological or morphological changes, i.e., wilting, yellowing, and shedding [12]. If the drastic water shortage cannot be controlled or reverted, dehydration will start to collapse leaf and stem cells and ultimately cause plant deaths and yield loss [13]. Crop yield is the lifeline of not only farmers but also humankind. Farmers have been concerned about agricultural drought ever since early farming society.