This section describes the main approach to this case study. The work of Lovett and Price (1999) has shown that the most important relationship between terrestrial ecosystem productivity and instream organic carbon fluxes is determined in the tributaries. However, the significant role of the
developed to define the functioning river ecosystems at catchment scale in terms of slow flow or quick flow and source of organic carbon inputs to the rivers (Vannote et al., 1980; Junk et al., 1989; Davis, 1994; Robertson et al., 1996; Lovett and Price, 1999). These models include: (1) River Continuum Concept; (2) Flood Pulse Concept; (3) A Hybrid Concept.
2.5.1 River Continuum Concept
The River Continuum Concept (Vannote et al., 1980; Minshall et al., 1985) was applied to a model of upland streams (tributaries). In this model, the lower stream reaches of organic carbon leach through upstream processes (Lovett and Price, 1999). Consequently, organic carbon inputs (DOC and POC) enter from litter and not from riparian vegetation or river-floodplain interactions. The middle order reaches are more important in this process than the first or last order tributaries. Thus, terrestrial litter is imported to the tributaries through lateral linkages (but not through floodplains), and is processed through heterotrophic organisms and then transported to the lower reaches or last order streams (main stem or main river). Stream order means classifications of streams based on their position in the channel network (Lovett and Price, 1999).
Strahler’s stream order classification attaches a numerical order to each stream or tributary segment (Lovett and Price, 1999). Figure 2.2 is a schematic model that shows the relationship between stream order differences and position in a landscape. Under this system a first order stream has no tributaries and when two streams of equal order mix, the section or segment downstream of the junction increases, in order. In other words, joining of two first order streams generates a second order, two third order streams produce a fourth order stream and so on. The middle order reaches or numbers (number 2) are the most important within the river continuum concept (and were used in the Cotter River Catchment case study to locate sampling sites).
Riparian functions and the impacts of the micro-catchments associated with a middle order stream vary in terms of stream size. This is also the case in the Cotter River Catchment. Hence, modelling techniques should take account of the particular characteristics of the river concerned based on the stream order classification of a catchment. Moreover, in the Cotter River Catchment many sources of tributaries are longer than the main stem or trunk (the Cotter River). Indeed, more of the total catchment area enters into small tributaries in comparison with the Cotter River itself.
Interestingly, most of the water transported through a source stream comes from local catchment runoff that is discharged as base-flow into the stream. This process is captured in rainfall-runoff models as a slow flow component. Along small tributaries dense riparian tree cover can shade the whole streambed leading to lower temperatures and further limitation on aquatic plant growth. In this situation, the primary source of organic carbon for tributaries (from litter) lies outside of the stream or is allochthonous (off-stream). So, the River Continuum Concept shows that the sources of organic carbon input to the catchment area vary between the upper tributaries and the main river stem.
Figure 2.2: A schematic model showing the relationship between stream order differences and position in landscape
Source: Lovett and Price, 1999
2.5.2 Flood Pulse Concept
The Flood Pulse Concept (Junk et al., 1989; Bayley, 1991) assumes floods are an important driver of river ecosystems (Robertson et al., 1996). This modelling approach focuses on the river- floodplain and quick flow discharge as an input source of organic carbon to the streams, besides that derived through upstream processes (Lovet and Price, 1999). In fact, this model assumes that the role of lateral linkages in lowland reaches is more important than that of upper-stream links (Robertson et al., 1996). Under this condition, floodplain and riparian vegetation in lower lands
stimulates microbial activity and decomposes litter on the forest floor in forested catchments like the Cotter River Catchment.
Of the seven sampling sites in the Cotter River Catchment, three (sites 500004, 500005, and 500006) were allocated according to the first approach or concept and the rest (sites 500001, 500002, 500003, and 500007) were chosen based on the second concept (see Figure 5.1 and Table 5.1 of Chapter 5).
2.5.3 A Hybrid or Coupled Concept
Considering the two previous model concepts, and the characteristics of the Cotter River Catchment, the modelling approach adopted for this study was a Hybrid or Coupled Concept approach. It is an integration of the River Continuum Concept in the upland reaches and the Flood Pulse Concept in the lowland reaches (Robertson et al., 1996). Accordingly, this coupled perspective does not ignore the importance of vertical (river-groundwater) linkages in some parts of the catchment (Sedell et al., 1989; Thorp and Delong, 1994; Robertson et al., 1996). Indeed, the dominant linkages (from litter in upland tributaries or lateral links in lowland streams) are reflected in the direction of gross primary productivity through litter inputs. Litter inputs are also affected by stream discharge and surface runoff with a steep gradient (high slope) or flat lands (floodplain).
The hybrid modelling approach was designed to meet several requirements.
First, the simulation model used should be able to predict the flow regime or streamflow discharge in micro-catchment areas. The IHACRES model (Jakeman and Hornberger, 1993; Croke and Jakeman, 2004) and the regionalization method (Croke and Norton, 2004) (see Chapter 4) were used to address this first stage as a delineation of both spatial and temporal variation (Robertson et al., 1996). The IHACRES model is a rainfall-runoff model that is applied to calibrate and simulate stream flow at gauged locations (sites with available streamflow records). The model parameters calculated through this simulation model are then extrapolated to un-sampled sub-catchments without streamflow records through a regionalization method using runoff coefficient, unit hydrograph and flow duration curves. These two procedures described above are combined to estimate streamflow discharge at different points in the sub-catchment area.
Second, the model used needs to be able to utilize quantitative water quality (DOC and POC), hydrological, climatic and ecological data in defining catchment responses in terms of hydrological
organic carbon sequestration. In this case study, these data were collected through an extensive field sampling program and the collection of historical data.
Third, the modelling approach should attempt to better represent the linkage between different reaches and sub-catchment areas (the 75 sub-catchments) by estimating patterns of spatial and temporal organic carbon variations due to changes in environmental input source (GPP) and output source (POC and DOC). This analysis was achieved for this study through application of a radiation use efficiency model for GPP estimation, and a combination approach consisting of the rainfall- runoff model, the regionalization method and component load estimation to calculate hydrological organic carbon fluxes. Then, a cross-correlation analysis and the relevant regression analysis were carried out to compare the two time series data sets (GPP and hydrological flux of organic carbon).
Given the above requirements, the following major analytical procedures were developed for the Cotter River Catchment case study:
a. Measurements of gross primary productivity or GPP as an input source of carbon across the 75 sub-catchments were made during this research (see Chapter 6) and then summed for the Cotter River Catchment. GPP was calculated using a radiation use efficiency model (Roderick et al.,
2001). The main variable was remotely sensed vegetation greenness obtained from the MODIS satellite on a monthly time step. Seventy five micro catchments were monitored over one year from July 2003 to June 2004, so that it was possible to estimate the magnitude of carbon production that is potentially available for processing, storage and transportation in the catchment through streams and tributaries. The monthly time step captured seasonal variation in climate and fire events. Many of the processes relevant to GPP calculation are controlled by the degree of alteration of vegetation dynamics and solar radiation as well as climatic and ecological variations (Field et al., 1995; Robertson et al., 1996; Schloss et al., 1999; Roderick et al., 2001; Huete et al., 2002; Potter et al.,
2003).
b. The transfer of organic carbon fluxes was measured as an output source across the catchment through the stream network using the rainfall-runoff hydrological model (IHACRES) to calibrate streamflow discharge at gauged stations. This analysis used hydrological and climatic sites in the Cotter River Catchment. Following this, a new regionalization method was developed to extrapolate streamflow to un-sampled sub-catchments. This method used runoff coefficient, unit hydrograph, and a base flow filter to predict surface runoff and flow discharge at ungauged stations and micro-
catchments. Then, the component load was estimated to calculate organic carbon sequestration at different spatial and temporal scales across the Cotter River Catchment.
To estimate the quantities of organic carbon transferred across the terrestrial-stream channel, water quality data (DOC and POC) were needed. Consequently, a sampling program was established in the Cotter River Catchment to sample instream DOC and POC from July 2003 to June 2004. Using the component load estimation approach that utilizes simulated streamflow values at un-sampled sub-catchments and organic carbon results derived from the sampling program, the hydrological flux of organic carbon was estimated in order to compare it with the GPP results.
c. The relationship between the input flux of GPP and the output flux of hydrological organic carbon based on the quantitative measurements derived from two previous steps were investigated using a cross correlation analysis. As noted earlier (section 2.3), a cross correlation analysis investigates the relationship between input flux and output flux and identifies delay time results from comparing two-time series data set. Subsequent regression analysis represents a power law relationship between input flux of GPP and output flux of hydrological organic carbon across the catchment. This relationship provided a predictive model to estimate the hydrological flux of organic carbon on a catchment level basis. Through a cross-correlation analysis the two sub- hypotheses and research questions were addressed.
This type of study can fill some of the knowledge gaps in carbon processing and flow at the global scale and could be used to gain a useful perspective on the relative importance of the hydrological flux of organic carbon between forested catchments and river networks to overall carbon cycling (Robertson et al., 1996). An overview of the methodology adopted is shown in Figure 2.3. In this diagram, GPP from catchment level vegetation and organic carbon produced in streams is determined as the targeted values. In this Figure, the relevant contributing chapters are also shown.
Figure 2.3: The relationship among the contributed chapters showing the methodology adopted in the Cotter River Catchment
Time series of sub- catchment estimates of GPP
(Chapter 6)
In-stream sampling of DOC and POC
(Chapter 5)
Historic time series of streamflow and climate
from gauged streams (Chapters 3 and 4)
IHACRES Rainfall-Runoff simulation model (Chapter 4)
Regionalization method to extrapolate streamflow discharge to un-sampled sub-catchments (Chapter 4) Organic carbon load estimates at un-sampled sub- catchments (Chapter 5) Cross correlation analysis (Chapter 7)