1. Streamflow (simulated) Streamflow (observed)
The Role of Spatial and Temporal Variability of Pan-Arctic River
Discharge and Surface Hydrologic Processes on Climate
Jennifer C. Adam1, Fengge Su1, Dennis P. Lettenmaier1, and Eric F. Wood2
Department of Civil and Environmental Engineering, Box , University of Washington, Seattle, WA
2. Department of Civil Engineering, Princeton University, Princeton, NJ, 08544 2 3
Calibration Issues
Changes in Streamflow
ABSTRACT
The export of freshwater to the Arctic Ocean plays a key role in both regional and global climates (e.g. via effects on the strength of the thermohaline circulation of the global ocean). The focus of this project is to assess the effects of changes (including spatial and seasonal variability as well as long-term trends) in river discharge of freshwater on high latitude climate using a coupled model system. Changes in snow cover extent over the pan-arctic domain have a direct effect on land-atmosphere energy exchanges, and also affect the seasonality of river flow. Furthermore, research has suggested that changes in permafrost extent and the active layer depth may also have significant effects on river flow. The work presented here partially satisfies Task 2 of the project (land surface model off-line simulations) in which the goals are to estimate the inflow to the Arctic Ocean from all pan-arctic land areas (including the Canadian Archipelago), to asses the capability of the land surface model to simulate the observed changes in gauged streamflow, and to use the model to evaluate the effects of changes in snow cover extent and active layer depth on streamflow variability. We report a 46-year ( ) run of the Variable Infiltration Capacity (VIC) macroscale hydrology model over the pan-arctic land domain. VIC is a semi-distributed grid-based model that parameterizes the processes occurring at the land-atmosphere interface and includes several recent improvements specific to cold-land regions. We present an estimate of the annual mean freshwater inflow to the Arctic Ocean for the 46-year period and preliminary analyses of streamflow and snow cover extent trends and variability.
Lena (at Kusur): 2,430,000 km2
Ob (at Salekhard): 2,950,000 km2
Yenesei (at Igarka): 2,440,000 km2
Mackenzie (at Arctic Red River): 1,680,000 km2
Yukon (at Pilot Station): 831,000 km2
Simulated
Observed
Pan-Arctic: 24,950,000 km2
Lena (at Kusur): 2,430,000 km2
Pan-Arctic: 24,950,000 km2
Simulated
Fitted
Simulated
Observed
The simulated discharge of freshwater to the Arctic Ocean varies between 0.18 and 0.25 million m3/s (5700 and 7900 km3/year) as shown in the figure in the upper left. A linear regression performed on the annual discharges yielded an increase of 610 m3/s/year (19.3 km3/year2) (Table 2). A linear regression of the annual precipitation over the pan-arctic (converted to similar units) yielded an increase of 356 m3/s/year (11.2 km3/year2), demonstrating that the increase in streamflow is mostly but not entirely due to an increase in precipitation. To evaluate the believability of this estimate, trends were determined for the observed and simulated streamflows for the five major basins and compared to trends of basin
The mean monthly hydrograph for pan-arctic discharge is shown for each decade in the above figure. This suggests that increases in precipitation (0.45 mm/year) and temperature (0.01 °C/year) lead to a slightly earlier melt, higher spring and summer discharges, and lower late summer through early fall and late winter discharges. 1
Modeling Framework
The hydrologic model is the VIC (Variable Infiltration Capacity) macroscale land surface model (Liang et al., 1994), which has been applied on a 100 km by 100 km EASE (equal-area projection) grid. Model features include multiple vegetation classes in each cell, energy and water budget closure at each time step, subgrid infiltration and runoff variability, and non-linear baseflow generation. VIC can be coupled off-line to a routing model in order to create a hydrograph at any location in a watershed. Recent cold-region developments in VIC include: improvements to the frozen soils algorithm to simulate permafrost; development of an algorithm to represent the hydrologic effects of lakes and wetlands; and development of an algorithm that estimates the redistribution and sublimation of blowing snow (Cherkauer et al, 2001 and Bowling et al, 2004).
Forcing variables are daily precipitation, maximum and minimum temperatures, and wind speed from NCEP Reanalysis. Precipitation monthly time-series are observation-based and corrected for gauge undercatch; Adam and Lettenmaier (2003). Temperature monthly time-series are from Mitchell et al., Daily variability for precipitation and temperature are based upon the work of Sheffield et al. (1994). Soil parameters are taken from the FAO global soil map. Land cover is from the University of Maryland 1-km Global Land Cover product (derived from AVHRR). Permafrost extent was defined using results from the 'frost index' permafrost model (Nelson and Outcalt, 1987).
average precipitation and temperature (Table 2). These results suggest that we are not simulating the trends correctly, e.g. the trend is of dissimilar magnitude and sign for the majority of the basins (e.g. the Lena in the figure above). The only variables that vary over time for this model setup are the daily meteorological forcings based on precipitation, temperature, and wind speed. For these five basins, changes in streamflow are balanced by increases in basin average precipitation. Either the forcings do not have homogeneous bias in time (esp. precipitation), or the model is not adequately capturing the phenomenon that is causing the observed streamflow trend. As a first step to investigating this, we are planning to improve our precipitation forcings by using GHCN station observations to correct for changes in bias with time using the method of Hamlet et al. (2004).
Table 2. Streamflow, precipitation and temperature trends.
Basin
Streamflow (simulated)
Streamflow (observed)
Precipitation
Mean Temp.
m3s-1/year
mm/year2
°C/year
Pan-arctic
610 NA
0.45
356
0.01
Lena
-36 19
-0.57
-44
0.02
Yenisei
-16 38
-0.34
-26 Ob 56 7
0.56 52
0.03
Mackenzie
207
-31
4.38
233
Yukon
192
6.83
180 4
Changes in Snow Cover Extent
The fraction of the pan-arctic domain covered with snow for each month is shown below (for each of the decades). Later decades exhibit slightly lower fractions throughout the spring but the fractions are not significantly changed the rest of the year.
Difference
Calibration was performed manually and focused on matching the shape of the monthly hydrograph through knowledge of the hydro-climatology of the various basins. The figures above show the comparison between simulated and observed (R-ArcticNET v3.0) streamflow at the mouths of the five largest basins. Work is ongoing to better match the hydrographs of some of these basins. The calibration period is 1979 through The relative root mean square error (RRMSE) and bias were calculated for each decade in order to evaluate the calibration outside the calibration period.
The number of days with snow cover per year was determined over the pan-arctic domain for each of the five decades. There are apparent differences between the first and last decades which are shown explicitly in the third of the series of figures ( subtracted from ). As expected, there is a decrease in number of days with snow cover for much of the basin, especially in the southern-most areas, but there is also a significant increase in the number of days with snow cover in much of the basin. This may be due to increased precipitation and is being investigated.
Definition of the pan-arctic domain is per ArcticRIMS and includes all land areas that discharge to the Arctic Ocean and Hudson Bay. Calibration of the soil file was performed over twelve basin groups (shown above). Work is ongoing to smooth the calibrated parameters across basin boundaries grid cells were included in the routing network (shown at right) in which 643 of these cells are outlets to the ocean. The routed outflow from these cells were summed to produce the pan-arctic discharge estimate, in which the contributing land area is approximately 25 million km2.
Table 1. Relative Root Mean Square (RRMSE) and bias statistics.
Basin
Statistic (%)
All Years
Lena
RRMSE
1.8
4.11
3.74
5.97
3.61
4.96
bias
11.26
18.47
8.67
16.48
6.6
3.15
Yenisei
1.6
3.2
3.8
3.66
3.45
3.57
7.86
9.87
14.42
7.6
6.48
-2.42 Ob
4.67
8.65
11.22
9.38
10.27
13.86
77.3
68.5
85.62
68.73
82.54
85.9
Mackenzie
6.4 NA
4.19
4.08
23.97
35.6
16.63
22.89
79.8
Yukon
5.9
10.59
5.84
15.04
-4.11
-17.47
15.6
22.03
CONCLUDING REMARKS
The simulated discharge of freshwater to the Arctic Ocean varies between 0.18 and 0.25 million m3/s (5700 and 7900 km3/year). The contributing area (draining to the Arctic Ocean and Hudson Bay) is approximately 25 million km2.
The inferred trend in discharge to the Arctic Ocean is 610 m3/s/year (19.3 km3/year2), although comparisons of observed and simulated streamflow trends for the major basins suggest that this estimate may be incorrect. Work to remove inhomogeneities from the forcing data is planned.
The work presented here is a partial completion of Task 2 of the project (off-line simulations). Work is underway to couple VIC to the climate model (Task 1) and to perform various one-way coupling sensitivity runs (Task 3).
Note: See the author for a list of references.