The time series plots presented on this page have been developed using the open water datasets corrected for cloud cover in combination with Central Valley Joint Venture (CVJV) Basin boundaries, a wetland layer developed by Ducks Unlimited (Petrik et al. 2014), and crop distribution layer derived from USDA’s Cropland Data Layers by The Nature Conservancy. Cloud filled images are overlaid on base layers for Basin, wetlands (seasonal and semi-permanent) and crops (corn and rice) to estimate the total area of open water by Basin and the proportion of open water by crop and wetland type overall across the Central Valley and in each Basin. Total area is the sum of all open water pixel values (1 or 0 where no clouds and between 0 and 1 [probability of water] in cloudy areas) in a Basin times the area of a single pixel (0.09 ha). The proportion of open water by cover types and Basin is calculated by first getting total area of open water by cover type and Basin and then dividing by the total amount of the respective cover type by Basin or for the full Central Valley.
More on cloud-filling
Total open water by CVJV Basin and the proportion of open water by cover type and cover type by Basin are calculated using cloud filled classified imagery. Clouds are a major problem for real-time datasets based on remote-sensing imagery. Landsat 8 only takes an image of any given area once every 16 days and so even a single cloudy day can, at the wrong time, delay an observation by over two weeks. Moreover, simply extrapolating the observed portion flooded to the unobserved (cloudy) area can result in wildly-fluctuating estimates of area flooded especially when the observed area is small. We developed a predictive model to fill areas of clouds. The model includes the average probability of open water by month over the previous 10-years for each pixel, the proportion of open water in the observable part of the Basin, and the Basin where the clouds occurred.
Petrik K, Fehringer D, Weverko A. 2014. Mapping seasonal managed and semi-permanent wetlands in the Central Valley of California. Ducks Unlimited, Inc. Rancho Cordova, California.