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4.0 Discussion
4.1 External Loads, Catchment Land Use Variation and the Importance of Sub- Catchment Scale Budget
This study’s goal was to conduct a water and nutrient mass balance, identifying the variance and relative importance of diferent nutrient sources and internal lake processes. For the purpose of this study, Pelican Lake’s watershed was
separated into multiple sub-catchments based on drainage and loading sources to the lake. The results identiied that external nutrient luxes into the lake were strongly linked to surface hydrology, and that variation in nutrient
concentrations was high between the tributaries that drained the lake’s sub-catchments. Eleven sub-catchments were identiied as possible loading sources, and Table 16 presents an estimate of contributing catchment yields over
the eight-month study period.
Catchment Area km
2
Percentage of Pelican Lake watershed
TP TN
Catchment yield kg·km
2
Catchment yield kg·km
2
PL05 25
2.1 211
606 PL08
700 60
81 34
PL09 5.2
0.5 60
343 PL10
16.9 1.5
31 197
Note: Catchment yield include stream inlow, diverted inlow and groundwater inputs.
Identifying critical source areas CSAs as a tool is important for addressing the management of phosphorus eutrophication and nutrient enrichment water quality issues, and is well documented in the literature Daniel,
Sharley, Lemunyon, 1998; McDowell, Biggs, Sharpley, Nguyen, 2004; Waters Webster-Brown, 2016. Sample site PL05 and its catchment was clearly the most signiicant contributor of external P load to Pelican Lake from
March to October 2016, in both contributing load and yield Table 14. Surprisingly, PL05 has a relatively small catchment area, 25 km
2
, compared to the other sites. For example, PL01 and PL08 have the largest catchment areas, 367 km2 and 700 km
2
, respectively. Input from PL08 is regulated by a dam, which would account for a lesser contribution; however, site PL01 low contributions were insigniicant to the lake’s mass balance, which
could be explained by unusually low precipitation observed in the region during 2016. The reasons for the high yield at PL05 are likely to include both source factors such as soil types and artiicial P sources fertilizer and
agricultural practices, and transport factors such as runof intensity due to ditch-culvert patterns, hillslope morphology, and erosion of hill slopes and stream banks Waters Webster-Brown, 2016. PL05, despite its relative
smaller catchment area size 25 km
2
, has the largest percentage of agricultural land use 82 per cent; Table 3. In comparison to PL09 and PL10, agricultural land use accounts for approximately 71 per cent and 45 per cent,
respectively. Pelican Lake’s phosphorous budget clearly indicated that targeted measures to control P transport should focus
on high-percentage agricultural land use catchment PL05, PL09 and PL10 and lood-associated transported at PL08-dam. The budget also illustrates that catchment area is not necessarily the strongest precursor to load
contribution and yield. The natural P-rich soils of the catchment and high agricultural land use suggest lake management should reduce stream and hill slope erosion, such as changes in soil, crop and stock management
Ekholm Lehtoranta, 2012. A more detailed understanding of speciic agricultural land use practices would increase the eiciency of targeting these transport processes.
Catchment yields were also estimated for the eight-month study period March to October 2016. Due to unusually low precipitation in 2016, it was assumed that the streams did not run or contribute signiicant low to the lake from
November to February precipitation as snow accumulated in ditches as snowpack; therefore, the catchment yields
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estimated for this study period are extrapolated as annual yields for comparison purposes to annual catchment yields found in the literature. Yields from North American mixed agricultural lands reported by Young et al. 1996
estimate 50–150 kg per km
2
per year and 10–70 kg per km
2
per year for south-east Australian pastures. Catchment yields are reported by Waters Webster-Brown 2016 at 68–81 kg per km
2
per year. Catchment yields for Pelican Lake are high, but several of the sites, such as PL09 and PL10, are not unusual for agricultural watersheds based on
the literature. In Pelican Lake, as in many prairie systems, the external loads of phosphorus and nitrogen are high and come
mostly via tributaries. The highest loads of nutrients come not from the largest catchment by area, but rather from the catchment with the highest percentage of agricultural area. This research found that surface low dominated
phosphorus inputs, and external inlux from each sub-catchment is strongly related to low. This result demonstrates the importance of variation in precipitation-related low associated with climate change variability and future
climate scenarios. Highly variable low regimes afect waterbodies in many ways, including impacts on stratiication, turbidity, and P load, and potentially afecting rates of internal loading North et al., 2015.
4.2 Flood Events on the Prairie Landscape and the Importance of Monitoring Spring Runoff Event
