Abstract Book - Soil and Water Conservation Society

Abstract Book - 68th International SWCS Annual Conference
49. Spatial Scale of Drought in a Meso-Scale Southwestern Watershed
Authors: Haiyan Wei*, University of Arizona; Mitchel Mcclaran, University of Arizona
Abstract: Large and intense droughts raise concerns about appropriate management responses, but
equally intense droughts may occur at smaller spatial scales, even when large droughts don’t occur.
We compare the spatial scale of drought between summer and winter seasons, and whether those patterns
changed after the recent dry period began in 1996. We use two measures of dryness, Standardized
Precipitation Index (SPI) and Palmer Drought Severity Index (PDSI) because the later includes temperature
and soil water holding capacity (SWC). We define drought as the lowest 20th percentile (driest 15
y) in the 73 y record (1940-2012); summer is Jun-Sep and winter is Dec-Mar; and the data represent the
225 km 2 Santa Rita Experimental Range in southern Arizona, using monthly records from 22 gauges,
temperature from PRISM, and SWC from a NRCS soil survey. SPI and PDSI were estimated separately for
one hundred 2.25 km 2 cells using an interpolated precipitation and area-weighted SWC. We compare
the frequency of small (22.5-45 km 2 ) versus medium (45-135 km 2 ) drought patches (contiguous cells).
Patch size was greater using PDSI than SPI, and small drought more frequent than medium drought using
SPI but less frequent using PDSI. Including the spatially uniform values of temperature in PDSI increased
the continuity of drought cells. Small patches were more frequent in summer than winter (4 vs. 1% of
years) using SPI, but less likely using PDSI (5 vs. 8%). Small droughts only occurred when the full-scale
(225 km 2 ) was not in drought. Medium patches were more frequent in summer than winter (15 v 5%)
using SPI, but less likely using PDSI (5 vs. 1%). More than 75% of medium patches occurred when the
full-scale area was not in drought. Since 1996, patch size increased using SPI and PDSI, and the summer
patches increased more than winter. Knowing that small areas of drought occur even when large areas
are not in drought will help design pasture size, herd movement, and drought detection efforts.
Subject Area: Rangeland Conservation and Grazinglands CEAP; Water Resources Research, Education,
and Outreach (NIFA Land Grand/Sea Grant 406 NRI)
*denotes primary author
Page 127