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Impacts of climate change on phosphorus loading from a grassland catchment
- implications for future management.

Eleanor Jennings1,2*, Norman Allott3, Donald C. Pierson4, Elliot M.
Schneiderman4, David Lenihan5, Patrick Samuelsson6 and David Taylor1.

1. Centre for the Environment, School of Natural Sciences, Trinity College
Dublin, Ireland.
2. Department of Applied Sciences, Dundalk Institute of Technology,
Dundalk, Ireland.
3. Department of Zoology and Centre for the Environment, School of Natural
Sciences, Trinity College Dublin, Ireland.
4. New York City Department of Environmental Protection, 71 Smith Avenue,
Kingston, NY 12401, USA.
5. Kerry County Council, County Buildings, Tralee, County Kerry, Ireland.
6. Rossby Centre, Swedish Meteorological and Hydrological Institute, 60176
Norrköping, Sweden.

* Corresponding author
Eleanor Jennings
Department of Applied Sciences, Dundalk Institute of Technology, Dublin
Rd., Dundalk, Ireland.
Email: eleanor.jennings@dkit.ie; Phone: +353 (0)42 9381804 Fax: +353
(0)42933 3505

Abstract
Dynamic modelling was used to quantify the impact of projected climate
change, and potential changes in population and land-use, on phosphorus (P)
export from a sub-catchment in SW Ireland using the Generalised Watershed
Loading Functions (GWLF) model. Overall the results indicated that the
increase in annual TP loads attributable to climate change was greater than
that from either population or land-use change, and therefore that future
climate variability will pose an increasingly significant threat to the
successful long-term implementation of catchment management initiatives.
The seasonal pattern in projected P export mirrored changes in streamflow,
with higher rates between January and April and lower rates in summer. The
potential reduction in export in summer was, however, negated when
increases in population were included in simulations. A change in the
slurry spreading period from that stipulated in national regulations to the
months between April and September could potentially mitigate against
future increases in dissolved P export in spring. The results indicate that
projected changes in climate should be included when undertaking modelling
exercises in support of decision making for catchment management plans.

Key words: climate change; phosphorus; catchment; modelling; GWLF; Ireland.
1. Introduction
The marked degradation in freshwater bodies over recent decades has led to
a more regulatory approach to their management in both Europe and the US.
In the US, for example, legislation has been enacted at both federal and
state level to develop Total Maximum Daily Loads (TMDLs) for pollutants of
concern, while in Europe, the Water Framework Directive (Directive
2000/60/EC) (WFD) is currently transforming the management of water
resources. The aim of the WFD is the achievement of good water quality
status, as defined in the Directive, for all water bodies. It requires that
management plans are implemented for all river basin districts (RBD) and
that these plans are reviewed on an on-going six-yearly basis.
Implementation of these measures is, however, taking place during a period
of unprecedented change in global climate, driven in part by human-induced
changes in atmospheric composition. Eleven of the warmest years in the
instrumental record have occurred between 1995 and 2006, while changing
trends in precipitation have also been observed in many regions, with
wetter conditions in Northern Europe and an increase in cyclonic activity
in the North Atlantic (IPCC, 2007). If no further action is taken to
reduce greenhouse gas emissions, the global average surface temperature is
likely to rise by a further 1.1-6.4°C this century (IPCC, 2007). Although
the WFD does not explicitly refer to climate change, future climate
variability has obvious implications for the long-term implementation of
the Directive and for the formulation and review of management plans (Wilby
et al., 2006; Ulen and Weyhenmeyer, 2007). The pressing need for more
studies investigating the potential impacts of these changes on freshwater
systems, in particular impacts on water quality and the coupling of climate
model output with land-use change, was emphasised in the latest IPCC report
(Kundzewicz et al., 2007). These studies can only be undertaken through a
combination of climate and catchment modelling.

One of the greatest pressures on water quality in freshwater systems in
recent decades has been excessive phosphorus (P) loading (Schindler, 2006).
While the export of nutrients to lakes from point sources, such as
municipal and industrial outflows, is independent of climate, transfers
from non-point or diffuse sources are highly sensitive to climatic factors.
In particular, the magnitude, and spatial and temporal patterns of nutrient
losses from catchments are directly governed by the spatial patterns and
intensity of precipitation (Donohue et al., 2005; Ulen et al, 2007). The
rate of P loss is also a function of long-term land use and management
(Daly et al., 2001; Cummins and Farrell 2003), together with catchment
population pressures (Edwards and Withers, 2007). Despite this, relatively
few studies have investigated the impact of climate change on P export from
European catchments and none have explored the combined impacts of changes
in climate, population, land-use and land management on P transfer.

In this paper, dynamic modelling is used to quantify the impact of
projected changes in catchment hydrology, together with a set of potential
changes in population and land-use, on P export from a sub-catchment in SW
Ireland and to assess the implications of these changes for catchment
management. The Generalised Watershed Loading Functions model (GWLF)
(Schneiderman et al., 2002) is used in the current research to simulate
streamflow, sediment yield and dissolved and total nutrient export. The
model version was developed by New York City Department of Environmental
Protection (NYC DEP) and was applied in a series of European catchments
during the EU-funded CLIME project to assess impacts of climate change on
dissolved nutrient losses (Moore et al., 2008; Pierson et al., in press,
Schneiderman et al., in press). In the first set of simulations, future
climate data are used to drive the model, while land-use, population and
management factors are kept at current levels to allow assessment of
weather-related impacts alone. The same future climate data are then used
to drive simulations representing a set of potential changes in population,
land-use and cattle slurry management.

2. Methods
2.1 Site description
The Lough Leane catchment (52o 05' N, 09o 36' W) (562 km2) comprises upland
mountain peat to the south and west and mainly agricultural grassland to
the east (Fig. 1). The main land-uses in the River Flesk sub-catchment (325
km2), the largest of three sub-catchments and the focus of the present
study, are intensive cattle farming, extensive sheep farming, and some
coniferous forestry. The area has a temperate oceanic climate due to its
proximity to the Atlantic Ocean. Rainfall varies considerably across the
catchment, from approximately 1000 mm year-1 in the northeast to 2700-3200
mm year-1 in the southwest (Allott et al., 2008). Lough Leane has a surface
area of 20 km2, a mean depth of 13.4 m and a retention time of 0.57 years.
The lake has undergone several changes in trophic status in recent decades
(Jennings and Allott, 2006; Jennings et al., 2008). Monitoring indicates
that the lake is still at the upper end of the mesotrophic classification
and therefore particularly sensitive to changes in nutrient loading whether
due to climatic or management factors. The Flesk sub-catchment is now
estimated to supply 70%-80% of the total P (TP) load to Lough Leane (Kirk
McClure Morton, 2003).

2.2 Model input data
Model driving data (daily precipitation and air temperature) were available
from a station at Muckross (Fig. 1). Land-use classification in the model
version is based on European CORINE level 3 land cover classes (Table 1).
Pasture is further divided into High, Mixed, and Low Productivity to allow
for differences in management. Parameter values for the Universal Soil Loss
Equation (USLE), used in the model to calculate erosion, were based on
Wischmeier and Smith (1978) (Table 1). An average slope length and slope
gradient were calculated for the USLE for each land class using a catchment
GIS and ordinance survey maps.

Soil nutrient concentrations were based on published values for Irish soils
(Table 1). A catchment average concentration was applied for the Roads
class. Land class specific estimates of dissolved P concentrations (Table
1) were based on molybdate reactive P (MRP) data from previous studies in
the Leane catchment (Kirk McClure Morton, 2003). A value of 0.42 mg
dissolved P L-1 was used in model runs for runoff events from High
Productivity Pasture during the slurry season, representing an average
relative increase in concentration (Bundy et al., 2001; Withers et al.,
2001; Kleinman et al., 2003; McGechan et al., 2005; Vadas et al., 2007).
The slurry spreading period was set to the dates stipulated in current
Irish regulations implementing good agricultural practice in Ireland (S.I.
378 , 2006) which prohibit spreading between 15th October and the
following 15th January.

Population data were obtained from the Irish Central Statistics Office.
Estimates of present-day tourist numbers were based on seas