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Water Storage Retrieval

Semi-supervised water storage retrieval using Sentinel-2 and in situ bathymetric data.

The pipeline is divided into three phases:

Phase I: Automatic Water Classification

Water is automatically classificated in an object-based approach using Simple Non-Iterative Clustering (SNIC) and k-means.

First, water is automatically classified in /phase-I/01-automatic-water-classification.ipynb. The Sentinel-2 image is preprocessed (median filtering is applied) and segmented using SNIC algorithm in Google Earth Engine. As inputs for the segmentation, the 10 m bands + the NDWI were used. Later, the segmented image is clustered using k-means. The cluster with the higher NDWI centroid is selected as the water cluster. The remaining clusters are labeled as non-water clusters.

Multiple hyperparameters were tested: Seed spacing and grid type for the SNIC algorithm, and the number of clusters and training size for the k-means algorithm. The results for each combination of hyperparameters were compared against actual water masks and statistical analysis were performed in /phase-I/02-statistical-analysis.Rmd.

For an automatic water mask, use the automaticWaterMask function in /phase-I/01-automatic-water-classification.ipynb.

automaticWaterMask(image, # Preprocessed Sentinel-2 image
                   ROI, # Region of Interest
                   index, # Index to use (NDWI or GNDVI)
                   seedSpacing, # Seed spacing for SNIC in pixels. 10 pixels is recommended
                   gridType, # Grid type for SNIC ("square" or "hex"). "square" is recommended.
                   scale, # Spatial resolution to work with. 10 for Sentinel-2.
                   k, # Number of clusters for k-means. 4 is recommended.
                   pTrain) # Training size as a fraction of the number of superpixels. 2 is recommended.

Phase II: Satellite Derived Bathymetry

Water depth is estimated through 3 different regression models: Linear Regression, Random Forest and Gradient Boosting.

First, reservoirs shorelines are retrieved and added to the original bathymetric data in /phase-II/01-shoreline-to-bathymetry.ipynb. The gathered data is interpolated using IDW in /phase-II/02-IDW-interpolation.Rmd. The input features for the regression models are calculated in /phase-II/03-input-features.ipynb. Regression models were fitted an tested in /phase-II/04-bathymetry-estimation.ipynb. Predictions were also made in this notebook. Statistical analysis of cross-validation results from the previous step were performed in /phase-II/05-statistical-analysis.Rmd.

Input Features

The input features for the regression models were the 10 m bands + Lyzenga Transforms + Ratio Transforms + Cumulative Cost Map.

Lyzenga Transform can be computed as the log transform of an specific band: log(x - xs), where x is the reflectance of the band x and xs is the average reflectance of deep waters in band x. For lakes and reservoirs, xs is set to zero.

Ratio Transform is calculated as the ratio of two log transformed bands: log(n*x)/log(n*y), where x is the reflectance of the band x, y is the reflectance of the band y and n is an user defined parameter. Here, n was set to 1.

The most important input feature for bathymetry estimation is the cumulative cost. This feature represents the euclidean distance from each pixel inside a water mask to the closest vertex in the shoreline of the water mask. Cumulative cost can be computed using the depthCumulativeCost function in /phase-II/03-input-features.ipynb.

depthCumulativeCost(waterMask, # Water mask (water mask derived from automaticWaterMask can be used here).
                    ROI, # Region of interest.
                    scale, # Spatial resolution to work with. 10 for Sentinel-2.
                    maxDistance) # Max distance to look for a closest point. 1000 is recommended.

Ensembles

Ensembles methods performed better than Linear Regression in bathymetry estimation. Random Forest and Gradient Boosting can be used. Best hyperparameters are showed below. Remaining hyperparameters are set to the default values in sklearn.

  1. Random Forest
from sklearn.ensemble import RandomForestRegressor
RandomForestRegressor(n_estimators = 20,
                      max_features = "sqrt")
  1. Gradient Boosting
from sklearn.ensemble import GradientBoostingRegressor
GradientBoostingRegressor(n_estimators = 500,                          
                          max_depth = 20,
                          max_features = "sqrt",
                          validation_fraction = 0.05, # Can be changed depending on your training size.
                          n_iter_no_change = 10,
                          tol = 0.01)

Phase III: Water Storage Retrieval

Total water storage volume is retrieved using the parameter-surface estimation method in /phase-III/01-water-storage-retrieval.ipynb. Bathymetry predictions of the three regression models were tested. Ensembles methods showed a better performance for water storage retrieval (RMSE < 4 hm^3).