19 Jul 2021

19 Jul 2021

Review status: a revised version of this preprint is currently under review for the journal ESD.

Vegetation indices as proxies for spatio-temporal variations in water availability in the Rio Santa valley (Peruvian Andes)

Lorenz Hänchen1, Cornelia Klein2, Fabien Maussion2, Wolfgang Gurgiser2, Pierluigi Calanca3, and Georg Wohlfahrt1 Lorenz Hänchen et al.
  • 1Department of Ecology, University of Innsbruck, Austria
  • 2Department of Atmospheric and Cryospheric Sciences, University of Innsbruck, Austria
  • 3Agroscope Institute for Sustainability Sciences ISS, Zürich, Switzerland

Abstract. 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 decreasing predictability of the onset of the rainy season and further challenges related to changes in rainfall characteristics. Previous studies based on time series of local rain gauges however, did not find any significant changes in either the timing or intensity of the wet season. Both in-situ and satellite rainfall data for the region lack the necessary spatial resolution to capture the highly variable rainfall distribution typical for complex terrain, and are often questionable in terms of quality and temporal consistency. To date, there remains considerable uncertainty in the RSB regarding hydrological changes over the last decades. In this study, we overcome this limitation by exploiting satellite-derived information on vegetation greenness to reveal a robust and highly resolved picture of recent changes in rainfall and vegetation phenology across the region: As the semi-arid climate causes water availability (i.e. precipitation) to be the key limiting factor for plant growth, patterns of precipitation occurrence and the seasonality of vegetation indices (VIs) are tightly coupled. Therefore, VIs can serve as an integrated proxy of rainfall. By combining MODIS Aqua and Terra VIs for 2000–2020 and several datasets of precipitation, we explore recent spatio-temporal changes in vegetation and water availability. Furthermore, we examine their links to El Niño Southern Oscillation (ENSO).

While different rainfall datasets tend to be incoherent in the period of observation, we find significant greening over the majority of the RSB domain in VI data, particularly pronounced during the dry season (Austral winter). This indicates an overall increase of plant available water over time. The rainy season onset and consequently the start of the growing season (SOS) exhibits high inter-annual variability and dominates the growing season length (LOS). The end of the growing season (EOS) is significantly delayed in the analysis which matches the observed dry-season greening. By partitioning the results into periods of three stages of ENSO (neutral, Niño, Niña), we find an earlier SOS and an overall increased season length in years associated with El Niño. However, the appearance of Niño/Niña events during the analysed period cannot explain the observed greening and delayed EOS.

While our study could not corroborate anecdotal evidence for recent changes in the SOS, 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.

Lorenz Hänchen et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esd-2021-60', Anonymous Referee #1, 21 Sep 2021
    • AC1: 'Reply on RC1', Lorenz Hänchen, 11 Oct 2021
  • RC2: 'Comment on esd-2021-60', Anonymous Referee #2, 02 Oct 2021
    • AC2: 'Reply on RC2', Lorenz Hänchen, 11 Oct 2021

Lorenz Hänchen et al.

Lorenz Hänchen et al.


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Short summary
To date, farmers perceptions of hydrological changes do not match analysis of meteorological data. In contrast, we find greening of vegetation, indicating water availability has increased in the past decades. The onset of the wet season however is highly variable, making farmers perceptions comprehensible. Hence, we show that El Niño Southern Oscillation has complex effects on vegetation seasonality but does not drive the greening we observe. Improved onset forecasts could help local farmers.