Changes in suspended sediment to solute yield ratios from an alpine basin during the transition to winter, southern alps, new zealand

Abstract

.Sediment yields from alpine environments are often assumed to significantly decrease during the transition from late summer to winter as declining temperatures reduce runoff and therefore sediment mobilization. We monitored suspended sediment and solute flux during this period in an 8.2 km2 catchment in the Southern Alps, New Zealand that contained a high elevation upper catchment and a smaller, low elevation subcatchment. Totals of 248 t of suspended sediment and 166 t of solute were exported over 91 days of measurement: a seasonal suspended sediment to solute yield ratio of 60:40 (or 1:0.67). This ratio differs from annual ratios of around 90:10 (or 1:0.11) reported elsewhere in the Southern Alps. Three phases of sediment transport were identified. Sediment yields in Phase 1 (28 days) and Phase 2 (30 days) were dominated by exports of suspended sediment (155 t) and less solute (94 t). Full winter conditions were experienced in Phase 3 (33 days) when upstream suspended sediment (17 t) and solute (29 t) yields decreased due to reduced discharge and freezing conditions. With this decline, the lower elevation subcatchment became the dominant runoff source and downstream suspended sediment (93 t) and solute (92 t) yields increased. We estimate that 20% of the annual solute yield was transferred during the transition to winter and 10% of the combined annual suspended sediment and solute yield. Overall, the results from this study suggest that suspended sediment to solute yield ratios are: 1) sensitive to seasonal changes in meteorological conditions, and 2) more significant during the transition to winter than previously recognised.