Spatial and temporal variations of water and nutrient fluxes within a steep-sloped agricultural catchment.

Abstract

A proper understanding of the spatial and temporal variations of runoff and nutrient fluxes are critical in understanding catchment hydrology. Runoff and nutrient fluxes may exhibit large variations both spatially and temporally, but this issue has largely been overlooked in the existing literature. The present study intends to respond to two main research objectives: (a) improve the understanding of the spatial and temporal variations (i.e. the dynamics) of overland flow (OF) and its factors of control and (b) quantify the evolution of runoff, nutrient and sediment fluxes from hillslope crest to catchment outlet. The research study was undertaken in a 1000 ha agricultural catchment of the Drakensberg foothills in the Bergville District, KwaZulu-Natal, South Africa under rangeland, small scale agriculture and commercial agriculture. The first objective was to evaluate the dynamics of OF during four rainfall seasons (2007 to 2011) by using 1×1m² microplots (n=15) located at five landscape positions within the rangeland upper part of the catchment. Automatic tipping buckets linked to a datalogger were used to estimate the delay between the start of the rain and the start of OF, which corresponded to the time of runoff initiation (TRI). Multivariate analysis was applied to the OF data and the information on selected environmental factors (rainfall characteristics, selected soil physical properties, soil water content and soil surface conditions). Nested scales of 1 and 10 m2 plots, and 23, 100 and 1000 ha catchments equipped with buckets for plots and conventional H-flumes for catchments, were used to quantify the downstream evolution of water and nutrient (C, NO3 - and P) fluxes. The fluxes were compared with data from the shallow and deep groundwater (GW) collected from piezometers and boreholes, respectively. This allowed for the determination of the mixing sources at the three catchment outlets, using stable isotopes of water (to differentiate between old and new water) and silica concentrations to identify soil water (SW) contributions. The average OF rate varied 2.3-fold across the Potshini Catchment (from 15% footslope to 35% backslope), while the average TRI varied by a 10.6-fold factor (between 0.6 minutes in the bottomland and 6.4 minutes at the footslope position). TRI temporal variations correlated the most with the duration of rainfall (Pearson r coefficient of 0.8) and the cumulative amount of rainfall after the onset of the rainy season (r=-0.47), while TRI spatial variations were significantly controlled by soil crusting (-0.97<r<-0.77). Water fluxes were found to increase iii from the microplot scale (208 l/m2) to the runoff plot scale (350 l/m2, delivery ratio of 1.68). The scale ratios calculated for the period of 2010-2011 show that there was a steady decrease in the delivery of water from the hillslope scale to the catchment scale. Cumulative water fluxes were found to be 316 l/m2 at the 23 ha catchment and 284 l/m2 at the 100 ha catchment (delivery ratios of 0.90 and 0.89 respectively). Water fluxes decreased sharply to 198 l/m2 at the 1000 ha catchment outlets (delivery ratio of 0.70). Runoff at the 23 ha catchment outlet was sourced from the mixing of GW (average of 63%), OF (22%) and SW (15%.) At the 100 ha outlet, GW contributions decreased to 50%, while OF contributions remained constant at 22% and SW contributions increased to 28%. The main contributor at the 1000 ha catchment was GW (55%) followed by SW (37%) and OF (8%). During the strongest rainfall event of the study period, OF contributed 97% to total runoff at the 23 ha catchment outlet, whilst at the 100 ha catchment, OF and SW both contributed 50% each. Groundwater in all cases was the major contributor to runoff at the 1000 ha catchment outlet. Both dissolved organic Carbon (DOC) and particulate organic Carbon (POC) increased from the microplot (8.44 and 25.51 g/m2 for DOC and POC) to the plot scale (14.92 and 26.91 g/m2). Lower yields occurred at the 23 ha catchment than on the hillslope (5.03 g/m2 and 8.18 g/m2). From the 23 and 100 ha catchment outlets, POC sharply decreased to 0.06 g/m2, while DOC increased considerably to 9.58 g/m2. This pointed to the decomposition of POC, which not only releases CO2 to the atmosphere but also adds DOC to runoff. At the 1000 ha catchment, POC yields were minimal due to a lack of eroded sediments whilst DOC decreased slightly (6.42 g/m2). These results yield a better understanding of the processes of water, nutrient and Carbon movements within landscapes. A further understanding of the processes leading to changes of nutrient and carbon fluxes needs to be performed in order to link this study with the overall ecosystem functioning of a landscape.