Comparative Analysis of Spatial-Temporal Trends of SPI and SPEI for the East River Watershed

Abstract:

The Standard Precipitation Index (SPI) and the Standard Precipitation-Evapotranspiration Index (SPEI) are multi-scalar parameters, which are often used to assess the spatial and temporal trends of climatic, i.e., drought-wetness, conditions. The goal of the presentation is to present the results of a comparative analysis of the long-term SPI and SPEI for meteorological stations located within a mountainous area of the East River Watershed, Colorado, as well as the data from the North American Regional Reanalysis (NARR) database. The SPI and SPEI time trends were calculated for various time scales-‑1, 3, 6, 12, and 24 months. The raw precipitation time series data are fitted to a gamma or a Pearson Type III distribution, transformed to a normal distribution, and then applied for calculations. It is shown that SPI is related to soil moisture dynamics on short timescales, whereas it can be related to the groundwater level dynamics across longer timescales. The SPEI was calculated to compare across watershed areas with noticeably different meteorological conditions. The SPEI was calculated using different potential evapotranspiration (PET) formulae for the same time scales as those for SPI. Unlike the SPI, SPEI captures the impact of changes in ambient temperature and other meteorological parameters on the water budget. PET was estimated with several methods--simple Thornthwaite and Hargreaves formulae, and a more sophisticated Penman-Monteith formula--to assess how various meteorological parameters, such as temperature, wind speed, air humidity and solar radiation, are accounted for in the evaluation of the SPEI and its temporal uncertainty. Statistical analysis was used to assess the uncertainty of calculations of SPI and SPEI for meteorological stations located at different parts of the East River watershed.