Understanding winter sodium deposition in Taranaki, New Zealand

Abstract

Research conducted in a limited number of regions has identified that Na deposition rate (kg Na/ha) is strongly influenced by 4 main factors: distance from coast, rainfall, wind speed, and wind direction. Despite the potential importance of Na deposition to the productivity of dairy farms, no comprehensive research has been conducted in Taranaki, New Zealand. Na, K, Ca, and Mg concentrations were determined in weekly rainwater samples collected in standard rain gauges erected at 15 sites, along 4 transects around Taranaki, between May and September 2006. Recorded Na concentrations ranged between 0.40 and 38 mg/L. High Na concentrations were associated with low rainfall volumes and proximity to the coast first receiving the prevailing wind, which was, during this period, the southern Taranaki coast. Na deposition ranged between 0.04 and 25 kg/ha.week. Equations were derived to predict the average Na concentration in rainwater and Na deposition in Taranaki for the 2006 winter period. The most influential factor explaining the variation in average Na concentration was the distance of the collector from the southern coast. Na and Mg depositions were highly correlated (R2 = 0.93; P < 0.01; n = 155), whereas correlations of Na with K or Ca were not as strong (R2 = 0.49 and 0.61, respectively). Measured Na deposition rates exceed those predicted by algorithms used in current nutrient budgeting software and could be used to improve this nutrient management software.