Widespread greening suggests increased dry-season plant water availability in the Rio Santa valley, Peruvian Andes
<p>In the semi-arid Peruvian Andes, the growing season is mostly determined by the timing of the onset and retreat of the wet season, to which annual crop yields are highly sensitive. Recently, local farmers in the Rio Santa basin (RSB) reported more erratic rainy season onsets and further cha...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-03-01
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| Series: | Earth System Dynamics |
| Online Access: | https://esd.copernicus.org/articles/13/595/2022/esd-13-595-2022.pdf |
| Summary: | <p>In the semi-arid Peruvian Andes, the growing season is mostly determined by the timing of the onset and retreat of the wet season, to which annual
crop yields are highly sensitive. Recently, local farmers in the Rio Santa basin (RSB) reported more erratic rainy season onsets and further
challenges related to changes in rainfall characteristics. Previous studies based on local rain gauges, however, did not find any significant
long-term rainfall changes, potentially linked to the scarce data basis and inherent difficulties in capturing the highly variable rainfall
distribution typical for complex mountain terrain. To date, there remains considerable uncertainty in the RSB regarding changes in plant-available
water over the last decades. In this study, we exploit satellite-derived information of high-resolution vegetation greenness as an integrated proxy to
derive variability and trends of plant water availability. By combining MODIS Aqua and Terra vegetation indices (VIs), datasets of precipitation (both for 2000–2020)
and soil moisture (since 2015), we explore recent spatio-temporal changes in the vegetation growing season. We find the Normalized Difference Vegetation Index (NDVI) to be coupled to soil
moisture on a sub-seasonal basis, while NDVI and rainfall only coincide on interannual timescales. Over 20 years, we find significant greening in
the RSB, particularly pronounced during the dry season (austral winter), indicating an overall increase in plant-available water over the past 2 decades. The start of the growing season (SOS) exhibits high interannual variability of up to 2 months compared to the end of the growing season
(EOS), which varies by up to 1 month, therefore dominating the variability of the growing season length (LOS). The EOS becomes significantly
delayed over the analysis period, matching the observed dry-season greening. While both in situ and gridded rainfall datasets show incoherent
changes in annual rainfall for the region, Climate Hazards InfraRed Precipitation with Station data (CHIRPS) rainfall suggests significant positive dry-season trends for 2 months coinciding with the most
pronounced greening. As the greening signal is strongly seasonal and reaches high altitudes on unglaciated valley slopes, we cannot link this signal
to water storage changes on timescales beyond one rainy season, making interannual rainfall variability the most likely driver. Exploring El
Niño–Southern Oscillation (ENSO) control on greening, we find an overall increased LOS linked to an earlier SOS in El Niño years, which
however cannot explain the observed greening and delayed EOS. While our study could not corroborate anecdotal evidence of recent changes, we confirm
that the SOS is highly variable and conclude that rainfed farming in the RSB would profit from future efforts being directed towards improving
medium-range forecasts of the rainy season onset.</p> |
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| ISSN: | 2190-4979 2190-4987 |