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Can spatial and temporal nutrient concentration variability be captured by catchment agro-geographical characteristics and water quality modelling?
Linköping University, Department of Physics, Chemistry and Biology, Biology. Linköping University, The Institute of Technology. (Wetland Ecology)
Swedish Meteorological and Hydrological Institute, SMHI.
Swedish Board of Agriculture (Jordbruksverket).
Gothenburg University.
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2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

In water management, source areas need to be identified and seasonal variability of nutrient flows assessed to facilitate design of cost-efficient mitigation programs. This study aimed at investigating to what degree sub-catchment spatial and temporal nutrient concentration variability could be captured by their agro-geographical characteristics and water quality modelling.

An agricultural catchment (160 km2) in Southeast Sweden was investigated with respect to source areas for phosphorus (P), nitrogen and particle losses. The specific aims were to 1) investigate the spatial variability of nutrient and particle concentrations and transport from different sub-catchments, 2) analyze if sub-catchment characteristics could explain differences in nutrient and particle concentration dynamics and overall nutrient losses, and 3) evaluate how well monitored temporal and spatial variability in nutrient concentrations could be simulated by a catchment model (HYPE). The purpose with the latter was to find recommendations for further model development and identify limitations for the use of catchment models in local water management.

Water flow was measured in two stations during 2009-2011. Grab samples were collected in synoptic sampling campaigns covering 10 sampling points during periods that represented various water flow regimes. Water samples were analyzed for total P (TP), dissolved phosphate (PO4-P), nitrate (NO3-N) and suspended matter (SUSP). The HYPE model was setup with the same detailed agro-geographical data as used for the statistical analyses of spatial and temporal correlations. The results showed that the sub-catchment variability of all measured nutrient concentrations were correlated with agro-geographical characteristics. All fractions of P concentrations were strongly correlated with soil type, whereas NO3-N concentrations were more related to crop factors. With regard to temporal dynamics of monitored concentrations, links to seasonality and water flow were more significant for NO3-N than for TP. Concentrations generated from the water quality model (HYPE) did not capture the subcatchment or temporal variability indicated from monitoring, particularly not for P concentrations. Neither did the modelled correlation between agro-geographical factors and concentrations correspond to that found for monitored concentrations. Some suggestions for model improvement were identified. Although water quality models are useful for local water management when it comes to modelling the impact of e.g. measures or climate change, our results suggest that their value might still be more limited when assessing variability on the subcatchment scale.

Place, publisher, year, edition, pages
2015.
National Category
Ecology Water Engineering Other Biological Topics
Identifiers
URN: urn:nbn:se:liu:diva-117971OAI: oai:DiVA.org:liu-117971DiVA: diva2:812525
Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2016-12-09Bibliographically approved
In thesis
1. Particulate phosphorus accumulation and net retention in constructed wetlands receiving agricultural runoff: Critical analysis of factors affecting retention estimates
Open this publication in new window or tab >>Particulate phosphorus accumulation and net retention in constructed wetlands receiving agricultural runoff: Critical analysis of factors affecting retention estimates
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Ansamling och fastläggning av partikelbunden fosfor i anlagda våtmarker på jordbruksmark : Faktorer som påverkar retentionsskattningar
Abstract [sv]

Övergödning är ett allvarligt miljöproblem, som bland annat orsakar omfattande blomningar av alger och blågrönbakterier. I söt- och brackvatten är fosfor ofta det mest begränsande näringsämnet för dessa organismer, varför en minskning av fosfortillförseln är nödvändig för att nå förbättringar. I Sverige beräknas jordbruket bidra med 44 % av fosforbelastningen till Egentliga Östersjön, och olika åtgärder för att minska fosforförlusterna från jordbruksmark tillämpas runtom i Sverige.

Våtmarker anläggs ofta för att fånga näringsämnen och partiklar från jordbruksmark innan de läcker ut i vattendrag och slutligen i Östersjön. Tidigare utvärderingar av anlagda våtmarker i Sverige har visat på en varierande och relativt låg fastläggning av fosfor. Osäkerheten kring dessa utvärderingar är dock ganska hög, och bottnar i kunskapsluckor både vad gäller processer för fastläggning och transport av fosfor från mindre jordbruksområden. I denna avhandling utreds därför hur anlagda våtmarker fungerar som fällor för jordpartiklar och partikelbunden fosfor i områden med höga fosforförluster.

Sju anlagda våtmarker i jordbruksområden med mycket lerjordar studerades, och mängden fosfor och partiklar som fastlades på botten varierade mycket mellan olika våtmarker (13-108 ton partiklar/ha/yr och 11-175 kg fosfor/ha/yr). De faktorer i området uppströms som var kopplade till fosforfastläggning var lutningen i området, markens lerhalt och innehåll av växttillgänglig fosfor samt områdets djurtäthet.

Resultat från fyra våtmarker visade på en hög resuspension (partiklar från botten virvlas tillbaka upp i vattnet), men en del av de uppvirvlade partiklarna kom troligtvis från erosion från våtmarkernas sidor och inte från det material som fastlades på botten. Man såg även indikationer på resuspension från vattenprover tagna i utloppet av en annan våtmark. Där var partikelbunden fosfor klart dominerande, vilket kan ha varit en konsekvens av resuspension från botten.

Variationerna av fosforkoncentrationer vid in- och utlopp i sju anlagda våtmarker studerades, för att kritiskt kunna granska tidigare retentionsskattningar. Det var stora variationer i sambanden mellan vattenflöde och fosforkoncentrationer mellan de olika våtmarkerna. De faktorer som påverkade sambandet mellan flöde och koncentration var 1) om det var varm eller kall årstid (d v s sommar eller vinter), 2) om det var högt eller lågt vattenflöde, samt 3) om inflödet bestod av ett dräneringsrör eller ett öppet dike/åfåra. I våtmarker med öppet dike som inflöde var flödes-koncentrationssambandet av totalfosfor negativt vid låga flöden och positivt vid höga flöden. De olika sambanden visar hur viktig sättet att provta vatten är, då inkommande mängd fosfor både kan över- och underskattas om man inte är medveten om sådana variationer. Vid automatisk provtagning styrd av flödesmätningen sker detta ofta i utloppet, men eftersom vattnet har en viss uppehållstid i våtmarken (speciellt sommartid) kan retentionsberäkningen påverkas av att all provtagningen styrs av flödet i utloppet.    

För att identifiera vilka områden som bidrar med mest näring och partiklar valdes ett stort område dominerat av jordbruksmark ut – för att undersöka hur man skulle kunna bedöma var anlagda våtmarker skulle kunna göra mest nytta. Området delades in i tio mindre områden, och vattenprover samlades in från diken och åfåror. Resultaten visade på stora skillnader i fosfordynamik mellan olika delområden med olika jordtyper, trots att de hade liknande markanvändning. Det fanns ofta en koppling mellan höga fosforkoncentrationer i vattnet och en hög andel lerjordar i området. För partiklar fanns det en tendens till samband mellan höga koncentrationer och hög andel vinterbar mark. En anlagd våtmark skulle antagligen ha högst effekt om den placerades nedströms områden som är känsliga för erosion – områden med hög andel lerjordar eller med hög andel vinterbar mark.

Abstract [en]

Eutrophication is one of the more serious current environmental problems, causing algal blooms and anoxic bottoms. In fresh and brackish water, phosphorus (P) is often the most limiting nutrient, and various mitigating strategies are used to reduce the load of P to sensitive recipients. In the agricultural sector, this includes both on-field measures (e.g. managing P inputs) and measures at the field edge (e.g. buffer zones and constructed wetlands). Previous evaluations of constructed wetlands (CWs) in Sweden have indicated a variable and relatively low P retention. However, the uncertainties in the estimates are large, and related to an incomplete knowledge about both retention processes and factors determining the P load from agricultural land. Hence, the overall aim of this thesis was to investigate possible reasons for the variation in wetland P retention estimates, and to assess the P retention in wetlands located in agricultural areas where losses are expected to be high.

When comparing seven CWs located downstream small catchments with predominantly arable land, the particle and P net accumulation varied considerably (13-108 t particles ha-1 yr-1 and 11-175 kg P ha-1 yr-1, respectively). Catchment factors that were statistically correlated with accumulation of particles and P in the CWs were the slope of the arable land, the P content of the top soil, the animal density (expressed as livestock units per arable land) and the percentage clay in the topsoils.

In four of the wetlands, resuspension was studied using sediment traps and plates. The results showed that up to 87 % of the settled material was resuspended, and indicated that erosion of the wetland sides and bottom probably contributed a substantial part of the trapped particles.

In order to critically evaluate existing retention data from earlier investigations, the temporal dynamics of P concentrations and P retention in seven CWs were evaluated. The relationships between water flow and concentration (from grab sampling) varied, and depended on the season (warm or cold period of the year), water flow (high or low) and the inlet type (drainage pipe or open ditch). In CWs that received water through an open ditch, flow-concentration relationships were negative during low flow periods but positive during high flow periods. These differences in flow-concentration relationship have implications for water sampling, since P loads can be both over- and underestimated with grab or automatic sampling guided by clock-time. Also composite automatic sampling, regulated from the water flow at the outlet, can lead to errors in transport calculations since the same ‘water parcel’ is not measured at in and out (difference depending on how long the water retention time is in the CW). This may have an effect on estimates of P retention in both past, present and future investigations of constructed wetlands.

Finally, a synoptic sampling approach with ten sampling points was used in an agriculturally dominated catchment area (160 km2) to identify differences in nutrient transport dynamics and areas with the highest losses. Spatial differences in P concentrations were strongly correlated with some of the catchment factors, for instance with soil type, and particle concentrations were weakly correlated to agricultural practices associated with bare soils during winter. This supports the practice to focus P mitigation measures – such as constructed wetlands – to erosion sensitive areas.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 49 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1648
Keyword
Phosphorus retention, constructed wetlands, agriculture, sampling strategy
National Category
Biological Sciences
Identifiers
urn:nbn:se:liu:diva-117116 (URN)10.3384/diss.diva-117116 (DOI)978-91-7519-107-2 (ISBN)
Public defence
2015-05-29, Planck, Fysikhuset, Campus Valla, Linköping, 10:00 (English)
Opponent
Supervisors
Available from: 2015-04-16 Created: 2015-04-16 Last updated: 2016-12-09Bibliographically approved

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