Limestone solution rates and processes in the Waitomo District, New Zealand

Abstract Karst solution processes are investigated on Oligocene limestones in the Waitomo district, west central North Island, New Zealand. Estimates of the inputs, throughputs and outputs of water and dissolved calcium and magnesium in two drainage basins were used to establish the rate of limestone solution by autogenic waters. The best estimate for solution loss from the basins during the study year is 69 m 3 /km 2 . The potential measurement errors inherent in each parameter used in the erosion rate computations were assessed and the probable maximum and minimum erosion rates were estimated to be 88 and 61 m 3 /km 2 . In both basins approximately 67 per cent of the annual solute load is transported by flows greater than the mean annual discharge, over 15 per cent being transported by flood flows that are exceeded only 5 per cent of the time. Almost half of the annual load is transported during the three winter months (June‐August), but no one month accounts for more than 18 per cent or less than 2.7 per cent of the annual total. Approximately 37 per cent of solution takes place within the soil profile, and most of the remainder is concentrated in 5–10 m of weathered bedrock (the subcutaneous zone) beneath this. Thus, it is likely that at least 85 per cent of the total solutional erosion contributes to the surface lowering of soil and bedrock.