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1 Slaets, J. I. F.; Schmitter, Petra; Hilger, T.; Vien, T. D.; Cadisch, G. 2016. Sediment trap efficiency of paddy fields at the watershed scale in a mountainous catchment in northwest Vietnam. Biogeosciences, 13:3267-3281. [doi: https://doi.org/10.5194/bg-13-3267-2016]
Sedimentation ; Paddy fields ; Watersheds ; Highlands ; Catchment areas ; Cropping systems ; Agriculture ; Maize ; Rice ; Irrigation water ; Soil fertility ; Erosion ; Reservoir storage ; Rain / southeast Asia / Vietnam
(Location: IWMI HQ Call no: e-copy only Record No: H047584)
http://www.biogeosciences.net/13/3267/2016/bg-13-3267-2016.pdf
https://vlibrary.iwmi.org/pdf/H047584.pdf
https://vlibrary.iwmi.org/pdf/H047584.pdf
(1.33 MB)
Composite agricultural systems with permanent maize cultivation in the uplands and irrigated rice in the valleys are very common in mountainous southeast Asia. The soil loss and fertility decline of the upland fields is well documented, but little is known about reallocation of these sediments within the landscape. In this study, a turbidity-based linear mixed model was used to quantify sediment inputs, from surface reservoir irrigation water and from direct overland flow, into a paddy area of 13 ha. Simultaneously, the sediment load exported from the rice fields was determined. Mid-infrared spectroscopy was applied to analyze sediment particle size. Our results showed that per year, 64Mgha-1 of sediments were imported into paddy fields, of which around 75% were delivered by irrigation water and the remainder by direct overland flow during rainfall events. Overland flow contributed one-third of the received sandy fraction, while irrigated sediments were predominantly silty. Overall, rice fields were a net sink for sediments, trapping 28Mgha-1 a-1 or almost half of total sediment inputs. As paddy outflow consisted almost exclusively of silt- and clay-sized material, 24Mgha-1 a-1 of the trapped amount of sediment was estimated to be sandy. Under continued intensive upland maize cultivation, such a sustained input of coarse material could jeopardize paddy soil fertility, puddling capacity and ultimately food security of the inhabitants of these mountainous areas. Preventing direct overland flow from entering the paddy fields, however, could reduce sand inputs by up to 34 %.
2 Slaets, J. I. F.; Schmitter, Petra; Hilger, T.; Hue, D. T. T.; Piepho, H. P.; Vien, T. D.; Cadisch, G. 2016. Sediment-associated organic carbon and nitrogen inputs from erosion and irrigation to rice fields in a mountainous watershed in Northwest Vietnam. Biogeochemistry, 129(1):93-113. [doi: https://doi.org/10.1007/s10533-016-0221-9]
Sedimentation ; Soil fertility ; Organic fertilizers ; Carbon ; Organic nitrogen compounds ; Nitrogen fertilizers ; Erosion ; Irrigation water ; Rice fields ; Maize ; Highlands ; Watersheds ; Nutrient balance ; Landscape ; Reservoirs ; Land use ; Runoff ; Shifting cultivation ; Surface water ; Water quality / Vietnam
(Location: IWMI HQ Call no: e-copy only Record No: H047588)
(1.27 MB)
Maintaining indigenous nutrient supply and positive nutrient balances are key factors in sustaining rice yields. Irrigation systems act as conveyers for water, sediments and nutrients throughout landscapes, especially in mountainous, cultivated tropical areas where erosivity is usually high. Contributions of erosion and irrigation to the nutrient balance of paddy fields, however, are rarely assessed. In this study, a turbidity-based method was used to quantify sediment-associated organic carbon and nitrogen as well as dissolved nitrogen inputs from erosion and irrigation to a 13 ha rice area in Northwest Vietnam. The irrigation source is a surface reservoir, and both reservoir and irrigation channel are surrounded by permanent upland maize cultivation on the steep slopes. Additionally, organic carbon and nitrogen loads in paddy outflow were determined to obtain nutrient budgets. Irrigation contributed 90 % of sediment-associated organic carbon inputs and virtually all nitrogen inputs. Analysis of ammonium and nitrate in total nitrogen loads showed that 24 % of the total N inputs from irrigation to the rice area, or 0.28 Mg ha-1 a-1, were plant-available. Loads measured at the outlet of rice fields showed that paddies were a trap for sediment-associated nutrients: balancing inputs and outflow, a net load of 1.09 Mg ha-1 a-1 of sediment-associated organic carbon and 0.68 Mg ha-1 a-1 of sediment-associated nitrogen remained in the rice fields. Sediment-associated organic carbon and nitrogen inputs thus form an important contribution to the indigenous nutrient supply of rice in these maize-paddy systems, while the rice fields simultaneously capture nutrients, protecting downstream areas from the effects of land use intensification on surrounding slopes. These results underscore the importance of upland-lowland linkages in tropical, mountainous, erosion-prone areas.
3 Slaets, J. I. F.; Piepho, H.-P.; Schmitter, Petra; Hilger, T.; Cadisch, G. 2017. Quantifying uncertainty on sediment loads using bootstrap confidence intervals. Hydrology and Earth System Sciences, 21:571-588. [doi: https://doi.org/10.5194/hess-21-571-2017]
Sediment ; Uncertainty ; Pollutants ; Nutrients ; Soil fertility ; Reservoirs ; Watersheds ; Water quality ; Discharges ; Catchment areas ; Rain ; Uncertainty ; Hydrological factors / Vietnam
(Location: IWMI HQ Call no: e-copy only Record No: H047983)
http://www.hydrol-earth-syst-sci.net/21/571/2017/hess-21-571-2017.pdf
https://vlibrary.iwmi.org/pdf/H047983.pdf
https://vlibrary.iwmi.org/pdf/H047983.pdf
(1.04 MB)
Load estimates are more informative than constituent concentrations alone, as they allow quantification of on- and off-site impacts of environmental processes concerning pollutants, nutrients and sediment, such as soil fertility loss, reservoir sedimentation and irrigation channel siltation. While statistical models used to predict constituent concentrations have been developed considerably over the last few years, measures of uncertainty on constituent loads are rarely reported. Loads are the product of two predictions, constituent concentration and discharge, integrated over a time period, which does not make it straightforward to produce a standard error or a confidence interval. In this paper, a linear mixed model is used to estimate sediment concentrations. A bootstrap method is then developed that accounts for the uncertainty in the concentration and discharge predictions, allowing temporal correlation in the constituent data, and can be used when data transformations are required. The method was tested for a small watershed in Northwest Vietnam for the period 2010–2011. The results showed that confidence intervals were asymmetric, with the highest uncertainty in the upper limit, and that a load of 6262 Mg year-1 had a 95 % confidence interval of (4331, 12 267) in 2010 and a load of 5543 Mg an interval of (3593, 8975) in 2011. Additionally, the approach demonstrated that direct estimates from the data were biased downwards compared to bootstrap median estimates. These results imply that constituent loads predicted from regression-type water quality models could frequently be underestimating sediment yields and their environmental impact.
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