My research project
Mapping historical hydrological changes in the lake Poopó catchment, Bolivia, with remote sensing
Lake Poopó, in the Andean Mountain range, has sustained local fauna, livestock and fishing activities of local communities for millennia. The lake is located at the bottom of an endorheic catchment where mining and crop activities have experienced a significant expansion in the last fifteen years. Several authors have reported a decline of the water storage in the lake during the last decade and highlighted different pressures affecting its water balance: diversion of water for mining and irrigation, the retreat of Andean glaciers, the droughts induced by El Niño event, etc.
As part of my PhD in the Civil Engineering Dep. of the University of Surrey, in collaboration with the Universidad Tecnica de Oruro and the Universidad Mayor de San Andrés, Bolivia, we are using long time series of satellite data to reconstruct spatial and temporal changes in vegetation, evapotranspiration water losses and precipitation over the Lake Poopó catchment, to explore their links to the lake decline.
This study undertakes a spatio-temporal assessment of the annual and seasonal precipitation trends over the Desaguadero-Poopó (DP) system in Bolivia for the period 1981-2018. Given the insufficient spatial density of meteorological stations in the study, the high-spatial resolution Climate Hazards group Infrared Precipitation with Stations (CHIRPS) satellite product was used. CHIRPS precipitation data was validated against 7,636 gauge measurements and used to map the spatial distribution of precipitation trends. In addition, the temporal changes in the onset, duration and average intensity of the wet season were explored. Results showed a good, statistically significant agreement between CHIRPS data and onsite records of monthly precipitation at the pixel and regional scales. The spatio-temporal precipitation assessment revealed that statistically significant increases in annual precipitation (2– 6 mm yr-1) occurred in two zones at the north and north-western part of the DP system. Only the southern part showed decreasing precipitation trends at a rate of -1 mm yr-1 while the central part did not show significant changes. Overall, a general increasing trend of about 2.5 mm yr-1 was observed for the entire DP system during the 38-year study period, with the months of December and February showing the highest significant increases. The analysis of the onset and cessation times of the rainy season indicated a statistically significant decrease in its duration at a rate of -0.4 day yr-1 and an increment of the average precipitation intensity. Our results reveal trend in the temporal concentration of precipitation in a water scarce region, which highlights the need for the management of the resource, so it can meet societal and environmental demands throughout the year.
Lake Poopó is located in the Andean Mountain Range Plateau or Altiplano. A general decline in the lake water level has been observed in the last two decades, coinciding roughly with an intensification of agriculture exploitation, such as quinoa crops. Several factors have been linked with the shrinkage of the lake, including climate change, increased irrigation, mining extraction and population growth. Being an endorheic catchment, evapotranspiration (ET) losses are expected to be the main water output mechanism and previous studies demonstrated ET increases using Earth observation (EO) data. In this study, we seek to build upon these earlier findings by analyzing an ET time series dataset of higher spatial and temporal resolution, in conjunction with land cover and precipitation data. More specifically, we performed a spatio-temporal analysis, focusing on wet and dry periods, that showed that ET changes occur primarily in the wet period, while the dry period is approximately stationary. An analysis of vegetation trends performed using 500 MODIS vegetation index products (NDVI) also showed an overall increasing trend during the wet period. Analysis of NDVI and ET across land cover types showed that only croplands had experienced an increase in NDVI and ET losses, while natural covers showed either constant or decreasing NDVI trends together with increases in ET. The larger increase in vegetation and ET losses over agricultural regions, strongly suggests that cropping practices exacerbated water losses in these areas. This quantification provides essential information for the sustainable planning of water resources and land uses in the catchment. Finally, we examined the spatio-temporal trends of the precipitation using the newly available Climate Hazards Group Infrared Precipitation with Stations (CHIRPS-v2) product, which we validated with onsite rainfall measurements. When integrated over the entire catchment, precipitation and ET showed an average increasing trend of 5.2 mm yr−1 and 4.3 mm yr−1, respectively. This result suggests that, despite the increased ET losses, the catchment-wide water storage should have been offset by the higher precipitation. However, this result is only applicable to the catchment-wide water balance, and the location of water may have been altered (e.g., by river abstractions or by the creation of impoundments) to the detriment of the Lake Poopó downstream.