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LIMITATIONS OF THE DROUGHT MONITORING INDEX FOR PREDICTING FORAGE PRODUCTION AND ADJUSTING LIVESTOCK NUMBERS.
Author
Perryman, Barry
Schultz, Brad
Stringham, Tamzen
Publisher
Society for Range Management
Publication Year
2016
Body

The Nevada BLM Drought Response Plans disclose the environmental impacts of implementing Drought Response Actions (DRA). We reviewed the usefulness and validity of the drought prediction tools and the DRAs. They have limited ability to serve as accurate and precise indicators of annual herbage production. Declaring drought based largely on Drought Monitor data misses the ecological principle that shallow rooted forage plants (grasses) do not depend on the amount of precipitation prior to the effective growing season, but rather the amount and periodicity of precipitation during the thermal growing season. When the drought monitor indicates drought in January, February or even March of the current year, that categorization may be totally irrelevant with regards to the effective moisture available during the growing season and total plant growth. The critical moisture variable for herbaceous forage production is the amount of soil moisture in the root zone when active growth begins, and for the subsequent 2-3 months. The timing and amount of precipitation that occurs in discrete precipitation events immediately before and during the growing season, not total precipitation, are critical factors for plant growth. Hydrologic drought, which largely reflects surface water supplies (i.e., the Drought Monitor) must be separated from growing season soil moisture drought, which largely influences forage production. Most (80%+) root biomass for perennial herbaceous species occurs in the top 16 to 20 inches of the soil. The water holding capacity in this depth zone for many soils is much less than the average winter and/or spring precipitation. One or two substantial precipitation events, correctly timed, can provide adequate soil moisture for abundant plant growth. We propose use of the Evaporative Demand Drought Index (EDDI) and similar tools in addition to current assessment methods to better capture wet and dry periods at both long and short time scales.

Language
English
Resource Type
Text
Document Type
Conference Proceedings
Conference Name
SRM Corpus Christi, TX
Collection
SRM Annual Meeting Abstracts