Abstract:
Diurnal variations in stream flow and stage levels have been observed in the Maraekakaho River in the Hawke's Bay region of New Zealand, along with other rivers in the area. In general, causes of diurnal variations in streams have been attributed to processes such as precipitation, evapotranspiration, infiltration (groundwater recharge), snowmelt (surface and subsurface runoff); processes part of the hydrologic cycle, and can also be induced by freezing and thawing processes, tidal effects and anthropogenic causes. Time series analysis of the Maraekakaho River and four other streams in the region exhibited maximum flow during the early hours of the morning and the minimum flow in the afternoon. The general trend of the shape of the diurnal cycle established was a gradual incline and a sharp decline. There was a distinct relationship with temperature with maximum stage or flow occurring at minimum temperature and vice versa. Also, larger differences in temperature meant that there was a larger difference between the daily maxima and minima. It is derived from these trends that the causes of diurnal variation in the Maraekakaho stream and the other four streams in the region are due to either evapotranspiration or infiltration, or a combination of both. MIKE SHE, a fully distributed physically based hydrologic model, was used to simulate the stream flow and groundwater flow observed in the Maraekakaho Stream to assess the suitability for modelling diurnal variations in rivers. Due to limited resources and time restrictions, the calibration and validation of the MIKE SHE model for this study was not successful. Many assumptions were made in the model which may have led to the incorrect representation of the catchment characteristics. Further research with a more comprehensive collection of data is recommended for the future.
