On improving cold region hydrological processes in the Canadian Land Surface Scheme
- First Online:
Regional and global climate model simulated streamflows for high-latitude regions show systematic biases, particularly in the timing and magnitude of spring peak flows. Though these biases could be related to the snow water equivalent and spring temperature biases in models, a good part of these biases is due to the unaccounted effects of non-uniform infiltration capacity of the frozen ground and other related processes. In this paper, the treatment of frozen water in the Canadian Land Surface Scheme (CLASS), which is used in the Canadian regional and global climate models, is modified to include fractional permeable area, supercooled liquid water and a new formulation for hydraulic conductivity. The impact of these modifications on the regional hydrology, particularly streamflow, is assessed by comparing three simulations performed with the original and two modified versions of CLASS, driven by atmospheric forcing data from the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis (ERA-Interim) for the 1990–2001 period over a northeast Canadian domain. The two modified versions of CLASS differ in the soil hydraulic conductivity and matric potential formulations, with one version being based on formulations from a previous study and the other one is newly proposed. Results suggest statistically significant decreases in infiltration and therefore soil moisture during the snowmelt season for the simulation with the new hydraulic conductivity and matric potential formulations and fractional permeable area concept compared to the original version of CLASS, which is also reflected in the increased spring surface runoff and streamflows in this simulation with modified CLASS over most of the study domain. The simulated spring peaks and their timing in this simulation are also in better agreement to those observed. This study thus demonstrates the importance of treatment of frozen water for realistic simulation of streamflows.
The Canadian Regional Climate Model (CRCM5; Martynov et al. 2013) has been applied in a number of studies to assess projected changes to streamflow characteristics, particularly for the high-latitude regions. The land surface model used in CRCM5 is the Canadian Land Surface Scheme (CLASS; Verseghy 2012), which is a physically based model. Studies that validated CRCM5/CLASS simulated spring streamflows have reported systematic biases (Poitras et al. 2011; Huziy et al. 2012). For example, the study by Huziy et al. (2012) suggests underestimation of spring peak flows for the province of Quebec. Difficulties in capturing the timing of spring peak flows were also reported in this study. These could be due to biases in the simulated winter precipitation and therefore snow water equivalent (SWE) and/or spring temperatures or due to simplified representation of land processes such as the frozen soil scheme in the model. For instance, not all land surface schemes take into account the possibility of having permeable and impermeable patches in a grid cell under frozen conditions. Stähli et al. (2001) demonstrated the important role of soil freezing-induced water redistribution on winter and spring runoffs using a land surface model with and without permeable fractional areas. Niu and Yang (2006) demonstrated significant improvements in the Community Land Model version 2.0 (CLM2.0) simulated spring streamflows with the inclusion of fractional permeable area during frozen soil conditions (i.e. partitioning the model grid into permeable and impermeable parts), representation of supercooled soil water (i.e. coexistence of liquid water with ice in the soil over a wide range of temperatures below 0 °C) through the freezing point depression equation and modified hydraulic conductivity formulation for frozen ground. According to Niu and Yang (2006), lack of representation of the above processes in the frozen soil scheme for their study regions led to low infiltration during snowmelt, leading to earlier and higher than observed springtime flows. Several other studies (Shanley and Chalmers 1999; Koren et al. 1999; Nyberg et al. 2001; Lindstrom et al. 2002; Cherkauer and Lettenmaier 2003; Bayard et al. 2005; Frampton et al. 2011) have also demonstrated the need to represent realistically frozen soil characteristics/processes to obtain realistic spring flows.
The main objective of this paper is to improve representation of the treatment of frozen water and thus runoffs and streamflow in CLASS by incorporating fractional permeable area, supercooled soil water, as suggested by Niu and Yang (2006), and a more realistic hydraulic conductivity formulation for frozen soil. Three simulations, one with the original and two with modified versions of CLASS, spanning the 1990–2001 period, are performed over a domain covering 21 selected northeast Canadian watersheds spread mainly across the province of Quebec and extending to some parts of Ontario and Newfoundland and Labrador provinces; the two modified versions of CLASS differ in their hydraulic conductivity and soil matric potential formulations which are discussed in details in Section 2. The study domain considered here is very important for hydroelectric power generation with almost 96 % of the total energy produced in the province of Quebec being hydro-based and therefore important for the economy of the province. A large number of hydro-related impact and adaptation studies for the region, in the context of a changing climate, rely on climate model outputs, including streamflows. Therefore, it is important to model streamflows realistically in climate models to have higher confidence in future projections. CRCM5 with CLASS as the land surface scheme and WATROUTE as the routing scheme is commonly used over the region and the proposed modifications to the treatment of frozen water are expected to improve the timing and magnitude of streamflows, particularly spring peak flows.
The article is organized as follows. An overview of the CLASS/WATROUTE framework and details of the experimental domain are provided in Section 2. Section 3 covers the theoretical basis of all methodologies related to fractional permeable areas, hydraulic conductivity and freezing point depression formulations for frozen soils considered in this article. Results from various experiments are presented in Section 4, followed by a brief summary of the results and conclusions in Section 5.
2 Model and experimental domain
2.1 CLASS/WATROUTE system
The basic function of the land surface scheme CLASS is to integrate the energy and water balances of the land surface forward in time from an initial starting point, making use of atmospheric forcing data to drive the simulation (Verseghy et al. 1993; Verseghy 2012). As the first approximation to subgrid-scale variability, CLASS adopts a “pseudo-mosaic” approach and divides the land fraction of each grid cell into a maximum of four sub-areas: bare soil, vegetation, snow over bare soil and snow with vegetation. The energy and water budget equations are first solved for each sub-area separately and then averaged over the grid cell, using averaged structural attributes and physiological properties of the four plant functional types (PFTs) in CLASS: needleleaf trees, broadleaf trees, crops and grasses. These structural attributes include leaf area index (LAI), roughness length, canopy mass and rooting depth, which have to be specified if they are present in a grid cell.
CLASS uses three soil layers, 0.1, 0.25 and 3.75 m thick in its standard formulation. The latest version of CLASS that is used in this study has a flexible soil-layering scheme, i.e. the total soil depth and the thickness of soil layers can be varied as desired. Since parts of the study region considered in this article are underlain by continuous/discontinuous permafrost, the soil depth is chosen to be 30 m with 20 soil layers to avoid inaccuracies stemming from the zero heat flux boundary condition at the lower model boundary. The thicknesses of the soil layers from top to bottom are 0.1, 0.2, 0.3 and 0.4 m followed by 0.5-m-thick layers. It must be noted that the hydrology calculations are performed only for soil layers above bedrock. In CLASS, the surface runoff for the modelled area is estimated from the excess ponded water, which varies with the land surface type. For example, for forested land, thresholds of 10 mm are used for ponded depth, above which runoff is generated. Free vertical drainage is assumed at the lower boundary of the permeable part of soil column, which is the sub-surface runoff. The CLASS-simulated total runoff (surface and sub-surface) is transformed into streamflows using the modified routing model WATROUTE (Soulis et al. 2000; Poitras et al. 2011). The routing scheme solves the water balance equation at each grid cell and relates the water storage to outflow from the grid cell using Manning’s equation. The routing model includes a groundwater reservoir, which is modelled as a linear reservoir as proposed in Sushama et al. (2004) and CLASS-simulated sub-surface runoff is used as input for the groundwater reservoir.
2.2 Experimental domain and geophysical fields
The watershed delineation, digital river network channel lengths and slopes required by the routing scheme at the model resolution of 0.44° are derived, following Huziy et al. (2012), based on the HydroSHEDS database (Lehner et al. 2008) that is available at 30-s resolution on latitude–longitude grid. In this study, WATROUTE uses daily time-steps and therefore runoff simulated by CLASS every 30 min are aggregated to daily intervals for use in WATROUTE for streamflow simulation.
As mentioned earlier, this paper focuses on improving the treatment of frozen water in CLASS with a view to improving simulated streamflows. Accordingly, three simulations are performed over the study domain: EXP-1 using CLASS with its original formulations, EXP-2 using CLASS with fractional permeable area, hydraulic conductivity and freezing point depression modifications as in Niu and Yang (2006) and EXP-3 using CLASS with a newly proposed parameterization for hydraulic conductivity and using fractional permeable area concept and supercooled liquid water as in EXP-2. The theoretical bases of all approaches are discussed in the section below. The initial soil conditions (i.e. soil moisture and temperature) were obtained by spinning up CLASS for 100 years using atmospheric forcing data from ERA-Interim for the 1971–1980, repeatedly, followed by a simulation for the 1971–2001 period and the results are analysed and presented in this article for the 1990–2001 period.
3.1 Theoretical bases
3.1.1 Experiment EXP-1
3.1.2 Experiment EXP-2
3.1.3 Experiment EXP-3
The above equation is derived using the approach of Campbell (1974) who estimated unsaturated hydraulic conductivity directly from a moisture retention function (i.e. Eq. (3)) using an integral equation that defines hydraulic conductivity as a function of pore radii. The details of the derivation of Eq. (10) are presented in Appendix I. As indicated earlier, this experiment also includes supercooled soil water, represented by Eq. (8), but with the modified matric potential (Eq. (9)).
Prior to studying the impact of various formulations discussed above on streamflows and other soil hydrological characteristics, the ERA-Interim data used for driving CLASS is validated to know what fraction of the biases in simulated streamflows can be attributed to biases in the driving data. To this end, the ERA-Interim seasonal mean temperature and precipitation (Dee et al. 2011) are compared to the gridded observation-based data from the Climate Research Unit (CRU) (Mitchell and Jones 2005) and University of Delaware (UDEL) (Willmott and Matsuura 1995) for the 1990–2001 period (see Section 4.1).
Description of the 21 watersheds and the gauging stations considered in the study area
Rivière à la Baleine
Réservoir Chutes Churchill
Rivière aux Feuilles
Grande Rivière de la Romaine
Complexe La Grande Rivière Sud
Rivière aux Melzèzes
Rivière des Outaouais
For northern parts of the study domain, observed snowmelt hydrographs generally have steeper rising limbs because of the low permeability of frozen soil. The proposed modification of hydraulic conductivity used in EXP-3 is expected to increase the steepness of the modelled rising limb of snowmelt hydrographs through increased runoff. This modification may also alter the recession limb indirectly by changing soil moisture, saturated/unsaturated water flows in the soil matrix and infiltration rate.
4 Results and discussion
4.1 Driving data validation
The positive temperature biases in ERA-Interim during DJF can lead to underestimation of SWE and thus have an impact on the simulated magnitudes of peak flows. Similarly, positive temperature biases during MAM can lead to earlier snowmelt and thus lead to biases in the timing of simulated streamflows. However, since the temperature biases in ERA-Interim are generally in the +2 to −2 range, except for some isolated grid cells, it can be hypothesized that improving the treatment of frozen water can indeed improve the quality of simulated flows.
4.2 Comparison of experiments
Biases in the snow water equivalent (SWE) in CLASS can have a huge impact on spring streamflows. The biases in SWE are therefore estimated by comparing maximum monthly SWE for the December–May period from different simulations, i.e. EXP-1, EXP-2 and EXP-3, with two gridded SWE products—GlobSnow (Global Snow Monitoring for Climate Research; Luojus et al. 2010) and CMC (the Canadian Meteorological Centre; Brown and Brasnett 2010) datasets. It must be noted that the GlobSnow product is derived from a combination of ground-based data and satellite microwave radiometer-based measurements, while CMC dataset is obtained from surface synoptic observations and meteorological aviation reports.
Soil moisture is compared between EXP-2/EXP-3 and EXP-1 for the May–August period, when the snowmelt rate and surface runoff are high (Fig. 4b). Soil moisture varies with processes such as infiltration of snowmelt/precipitation and contribution to subsurface flow in addition to evapotranspiration. Consistent with the lower hydraulic conductivity in EXP-3, soil moisture is significantly low in EXP-3 compared to EXP-1 for most of the grid cells in the southern and central parts of study area. EXP-2, on the other hand, exhibits lower soil moisture compared to EXP-1 despite the higher hydraulic conductivity. The lower soil moisture content in EXP-2 is due to increased drainage in EXP-2, which is the result of smaller absolute values of soil moisture potential in EXP-2 (Eq. (7)).
5 Summary and conclusions
Spring snowmelt peak flows in the high-latitude regions are often not realistically simulated by climate models, partly due to biases in the simulated temperatures and precipitation and partly due to deficiencies in the treatment of frozen water used in the land surface module of the climate model. In this paper, the sensitivity of simulated streamflows to selected modifications to the treatment of frozen water in the land surface model CLASS is investigated. Three offline experiments, EXP-1, EXP-2 and EXP-3, driven by ERA-Interim reanalysis, are performed with CLASS for the 1990–2001 period over a domain covering northeast Canada. Experiment EXP-1 is performed with the original version of CLASS, while EXP-2 and EXP-3 are performed with modified versions of CLASS. The fractional permeable area concept and supercooled liquid water are included in EXP-2 as in Niu and Yang (2006). This experiment also used the same hydraulic conductivity formulation proposed by Niu and Yang (2006). Experiment EXP-3 also includes fractional permeable area and supercooled liquid water but uses a more realistic formulation for soil matric potential and hydraulic conductivity. This yields lower hydraulic conductivity in EXP-3 compared to both EXP-1 and EXP-2.
Comparison of simulated streamflows to those observed at selected gauging stations suggests significant improvements in the magnitude and timing of spring peak flows in EXP-3. This is due to the reduced hydraulic conductivity of frozen soil in EXP-3, which leads to higher snowmelt runoff and smaller infiltration, which is reflected also in the reduced soil moisture content compared to EXP-1. EXP-2, on the other hand, it further reduces spring flows compared to EXP-1 due to the higher frozen soil hydraulic conductivity, leading to increased infiltration. However, the soil moisture does not increase in EXP-2 compared to EXP-1, despite the increase in infiltration. Analysis shows that this is due to the smaller absolute matric soil potential in EXP-2, which leads to increased drainage. Various statistical criteria considered to compare the experiments clearly suggest improved simulation of the timing and magnitude of spring peak flows in EXP-3. From the analysis performed in this study, it can be concluded that using a more realistic treatment of frozen water seems indispensable for simulating streamflows for high-latitude regions.
Experiment EXP-3, however, does not capture well the spring recession limb of the hydrograph and low flows in general. This can be improved through better presentation of hydrologic processes, such as surface-groundwater interaction in CLASS. Currently, the lower boundary condition is set to gravitational drainage in CLASS. Studies by Niu et al. (2007) and Yuan and Liang (2011) have shown the need to have improved representation of surface water-groundwater interactions to simulate better the soil moisture content and baseflow contributions to streamflows. Furthermore, significant proportion of streamflow contribution comes from lateral runoff or interflow in the case of summer storms (Letts et al. 2010). Interflow has not been included in the experiments presented in this study. Lack of interflow can partly explain the lower than observed streamflows in the simulations for the post snowmelt and summer seasons. Frampton et al. (2011) clearly demonstrates the important role of lateral flows and groundwater on streamflows. Inclusion of interflow and a groundwater component in CLASS is currently underway and this is expected to further improve CLASS-simulated runoff and streamflows for high-latitude regions. The simulations considered in this study did not include lakes and wetlands, which are also important for realistic simulation of streamflows. Furthermore, since CLASS uses a single layer representation of the snowpack, our future work will also focus on the inclusion of a multi-layer snow model in CLASS.
Though the formulations in EXP-3 improved spring flows in offline CLASS simulations, experiments will need to be conducted with the regional climate model with modified CLASS to see the impact of the modifications on streamflows in the coupled simulation since land-atmosphere interactions can lead to different results than those obtained in the offline simulations. The expectation is that the results would still hold since land-atmosphere interactions are generally weaker during the early phase of the snowmelt period.
- Brown R, Brasnett DB (2010) Canadian meteorological centre (CMC) daily snow depth analysis data. National Snow and Ice Data Center, BoulderGoogle Scholar
- Food and Agriculture Organization of the United Nations (2006) World reference base for soil resources, a framework for international classification, world soil resources reports, No. 103. FAO, RomeGoogle Scholar
- Hundecha Y, Zehe E, Bardossy A (2007) Regional parameter estimation from catchment properties prediction in ungauged basins, Predictions in Ungauged Basins: PUB Kick-off (Proceedings of the PUB Kick-off meeting held in Brasilia, 20–22 November 2002). IAHS Publ. 309Google Scholar
- Huziy O, Sushama L, Khaliq MN, Laprise R, Lehner B, Roy R (2012) Analysis of streamflow characteristics over north-eastern Canada in a changing climate. Clim Dyn 40(7–8):1879–1901Google Scholar
- Kulik VY (1978) Water infiltration into soil (in Russian). Gidrometeoizdat, 93 ppGoogle Scholar
- Luojus K, Pulliainen J, Takala M, Lemmetyinen J, Dersken C, Wang L (2010) Global Snow Monitoring for Climate Research-Snow Water Equivalent (SWE) product guide, European Space Agency: 15. ESRIN Contract 21703/08/I-ECGoogle Scholar
- Luthdin L (1990) Hydraulic properties in an operational model of frozen soil. J Hydrol 118(1–4):289–310Google Scholar
- United States Army Corps of Engineers (2000) Hydrological modelling system HEC-HMS, technical reference manual. United States Army Corps of Engineers, USA, 150 pp Google Scholar
- Verseghy D (2012) CLASS – The Canadian Land Surface Scheme (Version 3.6) Technical Documentation. 179 ppGoogle Scholar
- Webb RS, Rosenzweig CE, Levine ER (1991) A global data set of soil particle size properties. NASA Tech Memo 4286, 34 ppGoogle Scholar
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.