Distributed topographic indicators for predicting ... - Western University

Click Here

WATER RESOURCES RESEARCH, VOL. 45, W10407, doi:10.1029/2008WR007285, 2009

for

Full Article

Distributed topographic indicators for predicting nitrogen export from headwater catchments I. F. Creed1 and F. D. Beall2 Received 14 July 2008; revised 5 June 2009; accepted 2 July 2009; published 7 October 2009.

[1] The possibility of using topographic indicators to predict spatial variation in dissolved

nitrogen (N) export from headwater catchments was explored within a sugar maple forest in the Algoma Highlands of central Ontario, Canada, where the average annual export of total dissolved N export ranged from 3.58 to 6.96 kg N ha1 a1. Topographic indicators representing both ‘‘nondistributed’’ and ‘‘distributed’’ properties of the catchments were derived. Distributed topographic indicators that were designed to represent hydrologic flushing mechanism for N export were superior in predicting nitrateN export, explaining up to 85% in average annual nitrate-N export and 90% in the slope of discharge versus peak nitrate-N export which occurred during spring melt. However, the distributed topographic indicators were comparable to nondistributed ones for dissolved organic nitrogen export, explaining up to 68% of the variance compared to 65%. This study shows that spatial variation in N export from catchments within a relatively small region can be substantial, but that distributed topographic indicators can be used to predict a majority of this N export and thereby provide a basis for extrapolating N export from a few intensively monitored catchments to many other catchments within the sugar maple forest of the Algoma Highlands. Citation: Creed, I. F., and F. D. Beall (2009), Distributed topographic indicators for predicting nitrogen export from headwater catchments, Water Resour. Res., 45, W10407, doi:10.1029/2008WR007285.

1. Introduction [2] Emission control policies have been effective at reducing atmospheric sulphur deposition across eastern North America over the last several decades to the point that nitrogen (N) is almost the predominant acidifying pollutant [Jeffries, 1995; Jeffries et al., 2003]. To assess the potential impacts of acidic deposition and the need for further emission reductions many countries have adopted a critical load approach for setting regional limits for atmospheric N deposition (e.g., in Europe and Canada) [Burns et al., 2008]. The critical load approach is based on the idea that there is a theoretical maximum load of acidic deposition, referred to as the ‘‘critical load,’’ beyond which harmful effects can occur in terrestrial and aquatic ecosystems [Nilsson and Grennfelt, 1988]. Harmful effects are possible in terrestrial systems where chronically elevated levels of atmospheric N deposition have led to fundamental changes in N flows as systems shift from being anticipated net sinks for N to net sources of N (N-saturation) [Aber et al., 1989; Aber et al., 1998; Stoddard, 1994]. Potential limits for N emissions and the resulting N deposition may be evaluated by examining changes in the magnitude of N export to surface waters within a region. [3] Early in the development of the critical load concept, it was recognized that there would be considerable benefits 1

Department of Biology, University of Western Ontario, London, Ontario, Canada. 2 Natural Resources Canada, Sault Ste. Marie, Ontario, Canada. Copyright 2009 by the American Geophysical Union. 0043-1397/09/2008WR007285$09.00

gained in mapping critical loads [Gregor and Bull, 1988]. A comparison of maps of critical loads and deposition loads provides a method of identifying where the critical loads are exceeded and the degree of exceedance [Bull, 1992, 1995]. In mapping critical loads, a process of regionalization is required. This process of extrapolating from one place to another and scaling from finer to coarser scales necessitates a simplification of factors that go into the derivation of the critical loads, such as N cycling and routing processes. For example, once the regions are identified, estimates of critical loads may be achieved by extrapolating the biogeochemical response of a single intensively monitored catchment to other catchments in the region. In this approach there is a danger that the behavior of the intensively monitored catchment does not reflect the behavior of other catchments in the region, as there may be substantial temporal and spatial variation in the factors effecting N export [Lo¨vblad et al., 1995]. [4] Measurements of stream N concentrations and fluxes have been used widely to assess forest ecosystem response to a wide range of environmental stressors, including atmospheric deposition of acidifying substances, climate change, and forest management activities. What has emerged from these monitoring programs is that stream N concentrations and fluxes are highly variable both over time and across space. This variability arises from a multitude of abiotic and biotic processes influencing N storage, transformations, and transport. Some of the influences shown to affect stream N concentrations and fluxes include differences in forest type [Lovett et al., 2004], forest disturbance history [Goodale et al., 2000], hydrogeology and its effects on the source of water supplied to the stream [Schiff et al., 2002; Christopher

W10407

1 of 12

CREED AND BEALL: DISTRIBUTED TOPOGRAPHIC INDICATORS

W10407

W10407

Figure 1. Conceptual model of topographic influences on nitrogen export from forested catchments.

et al., 2008], and topography and its effects on hydrologic mechanisms that transport N to the stream [Creed and Band, 1998]. Generalization of factors influencing N export from forest ecosystems has been hampered by the substantial variability of N export observed from catchments within a forested region. [5] In this paper, we present an approach for generalizing topographic controls on hydrologically influenced N export from a few catchments that represent topographic variability to many headwater catchments within a forested region. Our objectives are to (1) establish the variation range of total annual N export, including the average annual flux of dissolved inorganic nitrogen (DIN (nitrate-N, ammonium-N)) and dissolved organic nitrogen (DON) among headwater catchments within the region, (2) derive topographic indicators of hydrologic features that are hypothesized to affect N export, and (3) develop statistical models that relate these topographic indicators to N export and thereby provide a means of extrapolating objectively the response of a few headwater catchments to other headwater catchments within the region. Our focus is on the forested region represented by a 10 km2 watershed in a sugar maple forest on the Algoma Highlands of central Ontario, Canada.

2. Conceptual Model [6] Our conceptual model (Figure 1) of topographic controls on hydrologically influenced N export (specifically nitrate-N) is based on a hydrologic flushing mechanism which has been shown to be an important mechanism of N export in our sugar maple forest [e.g., Creed et al., 1996]. Hydrologic flushing implies that N export is influenced by a

saturated throughflow process that is coupled to water table dynamics. When saturated throughflow is deep below the soil surface, N accumulates in the soil resulting in small export of N to surface waters. As saturated throughflow rises and intersects the soil surface, N formed at or near the surface of the soil is flushed resulting in large export of N to surface waters [Creed et al., 1996]. [7] Hydrologic flushing of nitrate-N may be influenced by topography through its effects on the source of flushable nitrate-N [Creed and Band, 1998]. Portions of the landscape may represent N-poor areas if the soil conditions inhibit nitrification (too dry) or promote denitrification (too wet). The critical transitional areas between uplands and wetlands may provide conditions that create nitrate-N rich areas, which are self-replenishing when hydrologic flushing is not active. Topography influences the hydrologic flushing mechanism through its effects on the transport of flushable N where N export is a function of both the size and the organization of variable source areas (VSAs). Catchments with larger, hydrologically organized VSAs will have larger N export. The hydrologic organization of VSAs is defined by their hydrologic connection to the surface drainage network. VSAs are highly organized if they are hydrologically connected to the surface drainage network such that hydrologic pulses transmit N efficiently to surface waters. In contrast, VSAs are disorganized if they are hydrologically disconnected (or isolated) from the surface drainage network such that N remains in the catchment or is leached to groundwater. [8] Hydrologic flushing of nitrate-N is also influenced by topography through its effects on the rate of expansion versus contraction of the VSA [Creed and Band, 1998]. As

2 of 12

W10407

CREED AND BEALL: DISTRIBUTED TOPOGRAPHIC INDICATORS

W10407

Figure 2. Location of the study site and the 13 experimental catchments. a surface saturating event proceeds, the newly saturated areas become the dominant source of the N flushing export as they represent areas where saturated throughflow is rising to the soil surface and flushing N from the soil to surface waters for the first time. Catchments with a greater potential for lateral expansion of source areas will have longer flushing times and higher rates of N export, while catchments with a lesser potential for lateral expansion of source areas will have shorter flushing times and lower rates of N export. [9] Nitrate-N flushed from uplands will not reach surface waters if it passes through topographic depressions and/or flats where it is transformed either into gaseous forms of N [Lindau et al., 1994; Ullah and Zinati, 2006] or into dissolved organic forms of N [Devito et al., 1989; Park et al., 2002; Blodau et al., 2006]. These topographic depressions and/or flats can therefore act as sinks for nitrate-N and sources of DON.

3. Study Area [10] The Turkey Lakes Watershed (TLW) (Figure 2) is an experimental forest (47°030000N and 84°2500000W) located on the Algoma Highlands on the northern edge of the Great Lakes – St. Lawrence Forest region, which is the second largest forested region in Canada. The climate is continental with average annual precipitation of 1200 mm and average annual temperature of 5.0°C. A snowpack persists from late November, early December through to late March, early April. Peak stream discharge occurs during snowmelt and again in September to November during autumn storms. [11] The watershed rests on Precambrian silicate greenstone formed from metamorphosed basalt, with small out-

crops of felsic igneous rock near Batchawana Lake and Little Turkey Lake. The overall relief of the TLW is 400 m, from 644 m above sea level at the summit of Batchawana Mountain to 244 m above sea level at the outlet to the Batchawana River. Overlying the bedrock is a thin and discontinuous till, ranging in depth from