Moving beyond the direction of climate change to estimating its magnitude: A water budget approach for wetland systems

Paleoclimate proxy studies of wetland sediment cores often document the direction if not the magnitude of climate change because transfer functions (pollen, diatoms, etc.) are unavailable or imprecise. A water-budget approach, presented here, circumvents the need for transfer functions in wetlands, using the Garden Basin cattail fen (GBC; Fish Lake Plateau, Utah, USA) as a model system. Prior studies of the GBC fen revealed both high and low-frequency oscillations between fen and pond states over the last 12.5 ka. Pollen indicated that pond-like conditions were accompanied by both greater monsoon and annual precipitation rates. However, the magnitude of these changes was not estimated. By synthesizing multiple approaches, including water-level monitoring, remote sensing (normalized difference vegetation index, NDVI), and terrain analysis, limits were placed on precipitation changes required for pond versus fen ecosystems in Garden Basin. NDVI data show that water levels may be controlled by evapotranspiration (ET) instead of spring snowmelt. Water levels increase in mid-winter in response to the decline of photosynthesis of surrounding pi pinon-juniper (PJ) forest. In late summer through fall, ET drives the fen system into a moisture deficit and water declines below the ground surface. Quantitative limits on both annual and seasonal precipitation changes can be obtained through water budgets. For the GBC model, mean annual precipitation (similar to 350 mm/yr) must double (similar to 700 mm/yr) to sustain perennial standing water for 95% of years. Increases in late summer (monsoon) through fall (cyclonic) precipitation requires lesser rates (100-200 mm/yr) to satisfy PJ ET demands to maintain standing water. Thus, the habitats of many wetlands globally may be particularly sensitive to late-season precipitation where ET is the primary control on water levels. In summary, the GBC fen and its water budget are a template for quantifying the magnitude, rather than just the direction, of precipitation change in wetland systems through time.