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This article is part of the Fall 2022 issue of the Earth Science Matters Newsletter. 

California farmers are the leading producers of many food crops in the United States, and their croplands are valuable to not only the United States and California but also to international markets. Since these croplands are so important and because drought and water management are top issues in California, it is crucial to monitor and understand the timing, size, and frequency of surface water inundation on these lands.

Inundation of croplands can occur from unintentional flooding, usually from weather-related events and thus less predictable and costly, or from intentional surface irrigation techniques. Inundating fields and relying on gravity-fed irrigation is still a common and inexpensive way to irrigate crops in California, although it has been in decline in favor of more water-conserving methods like drip irrigation. Further, some crops, like rice, require surface water inundation for several months during the growing process. However, unanticipated weather events, such as atmospheric rivers in California, can lead to flooding in sensitive croplands which leads to insurance claims and societal costs that are undesirable.

Insights into trends and patterns of inundation can be gained by using satellite imagery to examine surface water. Generating trends and patterns of inundation by crop type and county can be used to help farmers, resource managers, and policy makers develop resource use plans and mitigate flood damages. To better understand inundation patterns in California through time, a group of USGS researchers recently published a study that utilized Landsat and MODIS satellite imagery to classify surface water in croplands from 2003 to 2020.

satellite image of flooded rice fields
Water inundated rice fields, outlined in red, just west of Oroville, CA in May of 2019. In the false color image, purple hues indicate the presence of surface water where darker hues are deeper water. The bright green, yellow, and orange colors indicate areas not inundated by water. The true color image on the bottom is for comparison to show the usefulness of false color satellite imagery when distinguishing different land cover types. Image Credit: Image acquired using the European Space Agency’s Copernicus Sentinel 2B satellite, 25 May 2019.

Surface water was identified using the Dynamic Surface Water Extent algorithm, which classifies water into 4 categories (high confidence water, moderate confidence water, potential wetland, and low confidence water or wetland) based on likelihood of water presence. In this study, only high confidence (e.g., open water) classified water pixels were used for to represent monthly inundation across the time series (2003-2020). Those pixels were then intersected with 8 crop types (deciduous fruit and nut trees, vegetable, rice, field, grain, citrus, vineyards, and young perennial crops) from a 2016 statewide crop map generated by the California Department of Water Resources. Inundation for each county in California was summarized and exposure (total area inundated) and sensitivity (percent of crop type inundated) to flooding were examined for three crop types, deciduous fruit & nut trees, grape vineyards, and citrus crops. As a final step towards understanding the scale of unintentional cropland flooding over the study period, the United States Department of Agriculture’s Risk Management Agency’s crop insurance claim data were used to summarize spatial and monetary damages to different crops included in the study.

The research team found rice crops to generally have the highest amount of surface water inundation, which was unsurprising due to rice cultivation requirements, and citrus crops had the lowest annual and monthly occurrence of surface water inundation. On average, 50% of all cropland parcels showed signs of inundation across the 18-year timespan, which was generally attributed to intentional flooding for crop irrigation. Flooding exposure and sensitivity were geographically variable but were higher in counties of the southern Central Valley. Rice crops had the highest monetary flood and precipitation related insurance claims with damages generally occurring in the spring season. Deciduous fruit and nut crops also had high crop damages, which again generally occurred in the spring.

Surface water inundation maps are a valuable tool for CA stakeholders (farmers, government, etc.) interested in seeing either how field flooding is applied across different counties and crop types across the State, or those looking to gather insights into how vulnerable certain crops and counties are to unintentional flooding. This information can be applied to help guide future agricultural land use across California. By identifying the timing and location of flood prone areas, future flood damages and costs may be avoided either by avoiding farming in the risky area or by targeting the area for flood remediation efforts. On the flip side, identifying areas prone to being dry can help identify places well suited for crops like citrus and not great for crops like rice that require a lot of water. Ultimately, these results can assist state water managers examining or planning for surface irrigation needs to help focus efforts, use water more efficiently, and support conservation goals.

The paper, “Using Landsat and MODIS satellite collections to examine extent, timing, and potential impacts of surface water inundation in California croplands” was recently published in the journal Remote Sensing Applications: Society and Environment.

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