Description
Agriculture can be dynamic, especially in the arid Southwestern United States where water is a critical limiting resource. In 2003, the Quantification Settlement Agreement (QSA) established water conservation and ‘ag-to-urban’ transfer programs to reduce California’s use of Colorado River water. Initial water reductions to agriculture were achieved using a fallowing program (2003-2017). Changes in cropping activity during the fallowing program are unknown in California’s Imperial Valley. Existing annual crop classifications are too coarse to support intra-annual crop rotation research. Improved classifications are required to better understand multi-crop agricultural systems. In this thesis, one perspective from a tri-space conceptualization of Landsat time-series Normalized Difference Vegetation Index (NDVI) data is combined with the dimensionality reduction and analytical capabilities of the self-organizing map (SOM) method. A high-resolution SOM is generated from a 19-year NDVI dataset recorded with high temporal resolution. Clustering and visualization highlight unique NDVI phenologies and quarterly reference data (U.S. Bureau of Reclamation) identify clusters with known growing sequences. Sequences are modified to four categorical resolutions to classify original NDVI time-series data. Field activity in growing quarters was identified with an overall accuracy of 76%, 80%, 81%, and 90% at the growing sequence, group sequence, pattern sequence, and presence/absence sequence resolutions, respectively. Major transitions in growing activity are identified before, during, and after the QSA fallowing program. While most fallowed acres in 2002 (65%) or 2010 (52%) were still fallow in 2010 or 2018, demonstrating some persistence of fallowing, most of the remaining fallow transitioned to low-value crops. Most of the area planted in vegetables was previously alfalfa or bermuda grass, and most fallow was either alfalfa or bermuda grass, indicating the fallowing program mainly involved transitions among low-value crops with permanent transition for >50% of fallow fields. The rate of transition to high-value crops slowed from 2010-2018 compared with 2002-2010, due to more transitions back from vegetables to alfalfa. The results support an existing trend in the Lower Colorado River Basin of farmers transitioning to high-value, low-water-use cropping. Transitions in cropping activity are dynamic and could easily reverse depending on market conditions, constrained by rotational requirements that maintain field productivity.
