Low Spatial and Inter-Annual Variability in Evaporation from an Intensively Grazed Temperate Pasture System in New Zealand

Abstract:

Ecosystem scale measurements of evaporation (E) from intensively managed pasture systems are scarce and are important for informing water resource decision making, drought forecasting, and validation of Earth system models and remote sensing. We measured E from intensively grazed, unirrigated, ryegrass and clover pasture in New Zealand using eddy covariance (EC) for three years (2012 – 2014). Spatial variation in E was less than 3% during the initial study period when up to three sites were operating simultaneously. Inter-annual variability was also less than 3% over the three consecutive years (710 – 730 mm) at one site. The absence of spatial and inter-annul variation largely occurred because E was strongly controlled by net radiation (daytime half-hourly data r² = 0.83, p < 0.01) which was relatively consistent between sites and years. However, soil moisture decreased surface conductance during seasonal drought constraining E relative to net radiation. Variation in drought severity between years caused variation in seasonal E between years, for example, a relatively severe autumn drought in 2013 reduced E over autumn by 13% compared to 2012. Coincidentally, two unusually large spring and early summer rainfall events during warm conditions later in 2013 increased summer E by 12% compared to 2012 and therefore similar annual totals were measured between years. The FAO₅₆ Penman-Monteith model was able to accurately predict daily E over an annual cycle (r² = 0.81) to within 5 % of measured cumulative E with a crop factor of 0.96 (determined under non water-limiting conditions) and a water stress coefficient to account for soil moisture restrictions. Intensive grazing events, that remove a large fraction of standing pasture biomass, were found to have no effect on evaporation. The absence of a grazing effect suggests that leaf area was not an important control of E, likely because increases in soil E were able to compensate for decreased transpiration.