Spatial and Temporal Variability in Nitrate Concentration below the Root Zone in an Almond Orchard and its Implications for Potential Groundwater Contamination

Abstract:

Spatial and Temporal Variability in Nitrate Concentration below the Root Zone in an Almond Orchard and its Implications for Potential Groundwater Contamination

S. Baram¹, M. Read¹, D. Smart², T. Harter¹, J Hopmans¹

¹Department of Land, Air & Water Resources University of California Davis

²Department of Viticulture and Enology University of California Davis

Estimates of water and fertilizer losses below the root zone of nitrogen (N) intensive agricultural orchard crops are major concern in groundwater protection. However, microscopic and macroscopic heterogeneity in unsaturated soils make accurate loss estimates very challenging. In this study we aimed to examine field scale variability in nitrate (NO₃^-) losses below the root zone (>250cm) of a 15 years old almond orchard in Madera county California. Based on a soil variability survey, tensiometers and solution samplers were installed at 17 locations around the 40 acre orchard. The hydraulic potential and the NO₃^- concentrations were monitored over two growing seasons. Nitrate concentrations varied spatially and temporarily, and ranged from below to more than 30 times higher than the drinking water contamination standard of >10 mg NO₃^--N L^-1. Principal component analysis of the relations between the NO₃^- concentration, presence of a hard pan in the subsurface, its depth and thickness, and the fertigation and irrigation events indicated that none of these factors explained the observed variability in pore-water NO₃^- concentrations, with hard pan being the most dominant factor. Throughout the irrigation season minimal leaching was observed, yet post-harvest and preseason flooding events led to deep drainage. Due to the high spatial and temporal variability in the NO₃^- concentration and the potential for deep drainage following a wet winter or flooding event we conclude that the most efficient way to protect ground water is by transitioning to high frequency low nitrogen fertigation which would retain NO₃^-in the active root zone for longer time periods and thus enhance root absorption.