estimating trends in long-term evapotranspiration in the red river of the north basin, minnesota...
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Estimating Trends in Long-Term Evapotranspiration in the Red River of the North Basin, Minnesota and North DakotaPhil Gerla - University of North DakotaStudents in basic hydrology courses usually estimate evapotranspiration using methods appropriate for short-time rates at a point or in a small area on the landscape. Less often, however, do ET exercises use long-term and real data to test stationarity and estimate rates over large watersheds, which can be based on the simple steady-state form of the water budget equation: precipitation – surface-water runoff = ET. Anecdotal evidence suggests that during the last few decades, above-average precipitation and floods have occurred increasingly in the roughly 105 km2 Red River of the North basin. What is less clear, however, has been the effect of this apparent broad change of climate on the spatial and temporal variability of ET, which is a key component of the regional water budget. Has the ET rate and distribution been constant or has it changed? If it has changed, does it vary across the watershed? To answer these questions, both long-term precipitation and surface-water runoff data were tabulated and analyzed for subwatersheds in the region. This provided individual or small groups of hydrology students with experience in collecting and analyzing water-budget related data. The objectives of this exercise were to (1) work with large data sets from NWIS and NOAA, (2) use the U.S. Geological Survey's StreamStats to delineate watersheds and Climate Wizard to characterize changes in climate both forward and backward in time, (3) employ the water-budget equation to estimate ET for subwatersheds by using long-term records of precipitation and runoff, (4) apply the non-parametric Mann-Kendall test to characterize trends during the last 50 years, and finally, (5) relate the Wigley and Jones (1985) model to future stream runoff. Assessment of the exercise indicated some confusion and much angst among the students, suggesting the need for more background instruction and problem modularization. (Figure on the right shows the Mann-Kendall test probability for no trend in ET, (1965-2014) within sub-watersheds of the Red River basin).TRANSCRIPT
Estimating Regional Trends in Long-Term Evapotranspiration in the Red River of the North Basin, Minnesota and North DakotaPhil Gerla - H. Hamm School of Geology and Geological Engineering, University of North Dakota
Students in basic hydrology courses usually estimate evapotranspiration using methods appropriate for short-time rates at a point or in a small area on the landscape. Less often, however, do ET exercises use long-term and real data to test stationarity and estimate rates over large watersheds, which can be based on the simple steady-state form of the water budget equation: precipitation – surface-water runoff = ET.
Anecdotal evidence suggests that during the last few decades, above-average precipitation and floods have occurred increasingly in the roughly 105 km2 Red River of the North basin (map above). What is less clear, however, has been the effect of this apparent broad change of climate on the spatial and temporal variability of ET, which is a key component of the regional water budget. Has the ET rate and distribution been constant or has it changed? If it has changed, does it vary across the watershed?
left: Grand Forks, April 1997 (image from the U.S. Geological Survey)
above: map of the Red River basin above Lake Winnipeg (modified after Rasmussen 2015)
To answer these questions, students tabulated and analyzed both long-term precipitationand surface-water runoff data for subwatersheds in the region. This provided individual or small groups of hydrology students with experience in the following ways: (1) work with large data sets from NWIS and NOAA, (2) use the U.S. Geological Survey's StreamStats to delineate watersheds and Climate Wizard to characterize changes in climate, both forward and backward in time, (3) employ the water-budget equation to estimate ET for subwatersheds by using long-term records of precipitation and runoff, (4) apply the non-parametric Mann-Kendall test to characterize trends during the last 50 years, and finally, (5) relate the Wigley and Jones (1985) model to future stream runoff.
http://www7.ncdc.noaa.gov/CDO/CDODivisionalSelect.jsp#
http://streamstatsags.cr.usgs.gov/v3_beta/viewer.htm?stabbr=MN
Gilbert, R.O. 1987. Statistical Methods for Environmental Pollution Monitoring. Wiley Publishers. 320 p.Rasmussen, P.F. 2015. Assessing the impact of climate change on the frequency of floods in the Red River basin. Canadian Water Resources Journal, DOI: 10.1080/07011784.2015.1025101Wigley, T.M.L. & P.D. Jones. 1985. Influence of precipitation changes and direct CO2 effects on stream flow. Nature 314:149-151
Background Procedure and Data
Steady-state water balance is used to estimate ET from discharge (left) and precipitation (above) for a an assigned watershed. Watersheds are quickly delineated using USGS StreamStats (right):http://maps.waterdata.usgs.gov/mapper/index.html
Analysis
Students use the Mann-Kendall to test for trends (upper right) in their watershed, and probabilities assessed for all the sub-watersheds. Results can be integrated with global climate change models (ClimateWizard.org) and future conditions evaluated using Wigley and Jones (1985) model:
Assessment and ReferencesAssessment of the exercise indicated students quickly acquired knowledge of data extraction, but the analysis and interpretation led to some confusion and angst among the students and the instructor. This suggests the need for more background instruction and problem modularization, which will be developed and implemented in the future.
(Gilbert, 1987, includes probability tables)
w = runoff ratio, e = Δ ET, and p = Δ precipitation