Influence of hydrology on water quality and trophic state of irrigation reservoirs in Sri Lanka

Abstract

Many reservoirs provide multiple benefits to people around the world, in addition to primary uses such as irrigation. Thus, reservoir management should address their multiple uses. The water quality of ten irrigation reservoirs in Sri Lanka was examined in the present study with the objective of better understanding the effects of hydrological regimes on reservoir water quality and trophic state. Basic limnological parameters pertinent to the nutrient loads to, and trophic state of, the reservoirs were collected from June 2013 to February 2016. The sampling period was arbitrarily divided into two periods of approximately similar duration (period 1 = June 2013–September 2014; period 2 = October 2014–February 2016) to investigate whether or not there was a seasonal variation in the water quality parameters. Although temporal and spatial variations were observed, most water quality parameters were within the levels acceptable for drinking water standards. The 10 reservoirs were also ordinated by principal component analysis (PCA) on the basis of the water quality parameters of the two sampling periods in a two‐dimensional score plot. Reservoirs in the first principal component (PC1) axis were represented by negative scores attributable to the dissolved oxygen concentration and pH and, to a lesser extent, by electrical conductivity and chlorophyll‐a concentration. Positive scores in PC1 were represented by reservoirs with a score loading attributable to alkalinity, nitrate concentration, Secchi depth, temperature and seston weight and, to a lesser extent, from the total phosphorus concentration. There was a significant negative correlation of PC1 scores with relative reservoir water‐level fluctuation (RRLF; the ratio of mean reservoir water‐level amplitude to mean reservoir depth). Furthermore, Carlson's trophic index also were influenced by RRLF, although not by hydraulic retention time (HRT), indicating allochthonous nutrient inputs into the irrigation reservoirs were mainly governed by RRLF, but not by HRT. Thus, the results of the present study provide useful insights into achieving desirable reservoir water quality through the manipulation of the hydrological regime.

Patterns, Trends and Drivers of Water Transparency in Sri Lanka Using Landsat 8 Observations and Google Earth Engine

“…Results also revealed that the Z SD was generally highest during the SIM and NEM (October to February) seasons, which could reflect the occurrence of the highest precipitation, and on the contrarywise lowest during the driest SWM (May to September) season. These findings were similar to what other researchers have found at the reservoir scale [14,16]. Many studies reported that wind-driven sediment resuspension negatively impacts water transparency and is generally more prevalent among shallow reservoirs of Sri Lanka [14,56,57].…”

“…Direct influences are determined according to the population density [59], while indirect impacts are in the form of human-induced land-use changes, which influence sediment delivery, algal production, and vegetation cover [60,61]. According to the previous findings, population density has a significant influence on the rapid deterioration and degradation of the quality of surface water in Sri Lanka as the transparency is directly affected by the rate of sewage collection within the catchment [16,38]. This study also showed that the influence of the population was a significant factor in all zones.…”

“…Z SD is still widely in use locally [8][9][10] and globally [11,12]. In Sri Lanka, there have been a limited number of in-situ Z SD based studies reported at reservoirs or river basin scale [13][14][15][16][17]. However, so far, an island wide comprehensive investigation of water transparency distribution and its dynamics has not been executed and remains unknown.…”

Effects of hydrological regimes and limnological parameters on plankton community properties in tropical irrigation reservoirs: A Sri Lankan case study

“…Using 17 published food web models of 13 African lakes as a case study, Gownaris et al (2018) reported that water-level fluctuations are a key process influencing ecosystem structures and functions in lakes. Furthermore, in a parallel study the RRLF of the same 10 reservoirs in the present study was found to positively influence the reservoir trophic status (Nadarajah et al, 2019). Although biological F I G U R E 5 CCA loading plots for phytoplankton (•), 10 study reservoirs (∆) and physicochemical parameters (lines; reservoir abbreviations given in Figure 1; physical, chemical and biological parameters codes given in Table 2; phytoplankton taxa codes given in productivity-related water quality parameters exhibited some relationships with RRLF (Figure 4).…”

“…There is evidence that the hydrological regimes of irrigation reservoirs of Sri Lanka influence their water quality, trophic state (Nadarajah et al, 2019) and fish production (Amarasinghe & De Silva, 2015). The biological productivity and trophic status of reservoirs are known to be influenced by their hydraulic retention times (Nadarajah et al, 2019;Straškraba et al, 1995), the ratio of catchment area to reservoir extent (Nissanka et al, 2000) and the ratio of forest and shrub cover in the catchment to reservoir extent (Amarasinghe et al, 2002;De Silva et al, 2001). On the other hand, the WLF is a striking and regular feature of irrigation reservoirs because of the seasonal water releases for irrigation, which cause alternative littoral exposure and inundation.…”