E. D. Soulis

Towards closing the vertical water balance in Canadian atmospheric models: Coupling of the land surface scheme class with the distributed hydrological model watflood

Abstract Second generation land surface schemes are the subject of much development activity among atmospheric modellers. This work is aimed at, among other things, improving the representation of the soil water balance in order to simulate, more properly, exchanges with the atmosphere and to permit the use of model output to generate streamflow for model validation. The Canadian development program is centred on CLASS, the Canadian Land Surface Scheme, developed at Environment Canada. This paper focuses on the improvement of hydrology in CLASS. This was accomplished by designing a two‐way interface to WATFLOOD, a distributed hydrologic model developed at the University of Waterloo. The two models share many features, which facilitated the coupling procedure. The interface retains the three‐layer vertical moisture budget representation in CLASS but adds three horizontal runoff possibilities. Runoff from the surface water follows Mannings equation for overland flow. Interflow is generated from the near‐surface soil layer using a parametrization of Richards equation and base flow is produced by Darcian flow from the bottom of layer 3. An approximation of the internal topography of grid elements is used to supply horizontal gradients for the runoff components. Tests are in progress in four Canadian study areas. Initial results are presented for the summer of 1993 for the Saugeen River in southwestern Ontario. The new scheme produces realistic hydrographs, whereas the old scheme did not. Bare ground evaporation is reduced by about 17% as a consequence of reduced water availability in layer 1. Evapotranspiration is not affected because the rooting depth extends into layer 3, in which soil moisture does not change appreciably with the new scheme. These results suggest that the new scheme improves the representation of streamflow in WATFLOOD/CLASS and of the soil moisture budget in CLASS. Work is in progress to validate this result over basins, such as the BOREAS study watersheds, where both runoff and evapotranspiration measurements are available.

A Land Cover-Based Snow Cover Representation for Distributed Hydrologic Models

A snow cover depletion curve (SDC) summarizes the relationship between snow cover distribution and an average snow cover property, such as depth or water equivalent, for a given area. Snow cover depletion curves have been developed for, and applied in, hydrological models on a watershed or elevation zone basis. However, land cover-;based SDCs are not prominent in the literature. For this study the areal distribution of snow cover for dominant land cover units was measured during the winters of 1991 and 1992 in the Laurel Creek watershed in southern Ontario, Canada. On the basis of these data a general model for land cover-;based SDCs is developed for these land cover units, namely, short grass, ploughed fields, and deciduous forests. This model is derived from the three-parameter lognormal distribution, which is shown to characterize the areal depletion curves of the land cover units studied. The SDCs based on this new model provide a formal distributed snow cover representation that can be used in vegetation-based distributed hydrological models requiring accurate spatial representations of snow cover attributes.

Implications during transitional periods of improvements to the snow processes in the land surface scheme ‐ hydrological model WATCLASS

Abstract The representation of snow processes is crucial in both hydrological models and land surface schemes. The importance of the detailed physical representation of four snow processes in the WATCLASS hydrological‐ land surface scheme model is examined. The snow processes are: the occurrence of mixed precipitation; the density of fresh snow; the maximum snowpack density; and canopy snowfall interception. It is shown that the inclusion of the non‐static processes does not significantly improve the simulated streamflow. The changes in the simulation of state variables, in particular, the snowpack depth, snow water equivalent, soil temperature and soil moisture content are small, but may become important during transitional periods, such as the initial accumulation and depletion of snow‐covered areas during snowmelt. This substantially alters the surface heat fluxes during these periods.

MODELING THE RAINFALL-RUNOFF RESPONSE OF A HEADWATER WETLAND

In the eastern temperate region of North America, treed headwater swamps are a familiar watershed feature. These low-gradient wetlands commonly exist at groundwater discharge sites and represent a link between the underlying groundwater system and the surface drainage system. In contrast to the extensive literature pertaining to the hydrologic modeling of agricultural and forest land classes, little attention has been focused on the development and testing of numerical simulation models for predicting the hourly stormflow response from headwater wetland sites. If required to predict the rate of outflow from a wetland-dominated catchment, the hydrologist or engineer has few numerical tools and little data available to assist in the prediction. The objective of this research was to investigate the feasibility of applying a numerical model to simulate the rainfall-runoff response from a treed headwater wetland site. The wetland model utilizes a hydrology model coupled to a hydraulic stream-routing model. A depth-averaged laminar flow model is used to simulate the horizontal movement of stormwater both through and over the wetland sediments. The development and testing of the wetland model were completed in conjunction with a data collection program in which hydrometric and meteorologic data were obtained at a 400-ha first-order headwater swamp located within the Teeswater River watershed in southern Ontario, Canada. An analysis revealed that the simulated wetland streamflows were sensitive to the antecedent saturation of the wetland sediments, the storage and flow transport characteristics of the wetland sediments, and the conveyance capabilities of the wetland channel system.

Closing the Mackenzie basin water budget, water years 1994/95 to 1996/97

Abstract A particularly elusive science objective for the Mackenzie Global Energy and Water Cycle Experiment (GEWEX) Study (MAGS) has been to close the atmospheric moisture budget and rationalize it against the surface water budget at annual or even monthly timescales. The task, while not difficult in principle, is complicated by two factors. First is the importance of basin snow‐cover, soil and water‐body storage in the surface water budget. Month‐to‐month changes in these components are frequently greater than the atmospheric flux terms, for example, during spring snowmelt. Furthermore, there is approximately a six‐week lag before local changes are evident in the discharge at the mouth of the basin. Second, the coarse resolution of all of the supporting data may add significant systematic errors. For example, the two radiosonde soundings per day available to the project are unlikely to account adequately for all the moisture generated locally through evapotranspiration during the summer convective season. This analysis will directly address these two main issues by applying hydrologic and atmospheric computations to assess the storage question, and by using additional soundings at a single site to sample the diurnal signature in atmospheric moisture caused by evapotranspiration. Resulting modifications to the atmospheric moisture and surface water budgets then allow near closure of the MAGS monthly water budget within acceptable error limits.

The MAGS Integrated Modeling System

The Mackenzie GEWEX Study (MAGS) integrated modeling system was developed to couple, with full feedback, selected atmospheric and hydrologic models, with the expectation that the imposed consistency will enhance the performance of both models and so mitigate the lack of data for northern basins. As each modeling community moved towards using a common land surface scheme based on the Canadian land surface scheme CLASS, a new mesoscale distributed hydrologic model (WATCLASS) was created, using CLASS for vertical processes and the routing algorithms from WATFLOOD. The version of CLASS used in the atmospheric models was modified to reflect the experience with WATCLASS. Changes were made primarily to the soil water budget and included improvements in the between-layer transfer procedures, the addition of lateral flow, and the enhancement of the treatment of cold soil. The drainage database for the Mackenzie River Basin (MRB) was built from GTOPO-30 digital elevation model and the CCRS-II AVHRR-based landcover classification. Streamflow simulations using the WATCLASS model are compared to measured values for both the MAGS research basins and the major tributaries of the Mackenzie. As well as streamflow, simulated internal state variables from WATCLASS are compared to detailed measurements taken in the research basins. Finally, the water balance of the MRB is examined and the change in storage within the basin is compared to satellite data.

Calibrating Environment Canada's MESH Modelling System over the Great Lakes Basin

Abstract This paper reports on recent progress towards improved predictions of a land surface-hydrological modelling system, Modélisation Environmentale–Surface et Hydrologie (MESH), via its calibration over the Laurentian Great Lakes Basin. Accordingly, a “global” calibration strategy is utilized in which parameters for all land class types are calibrated simultaneously to a number of sub-basins and then validated in time and space. Model performance was evaluated based on four performance metrics, including the Nash-Sutcliffe (NS) coefficient and simulated compared with observed hydrographs. Results from two calibration approaches indicate that in the model validation mode, the global strategy generates better results than an alternative calibration strategy, referred to as the “individual” strategy, in which parameters are calibrated individually to a single sub-basin with a dominant land type and then validated in a different sub-basin with the same dominant land type. The global calibration strategy was relatively successful despite the large number of calibration parameters (51) and relatively small number of model evaluations (1000) used in the automatic calibration procedure. The NS values for spatial validation range from 0.10 to 0.72 with a median of 0.41 for the 15 sub-basins considered. Results also confirm that a careful model calibration and validation is needed before any application of the model.

The specific surface area of fresh dendritic snow crystals

The surface area to mass ratio or specific surface area (SSA) is an often neglected characteristic of the snowpack that varies substantially with time, and with the shape of the individual snow crystal for fresh snow. The SSA for the dendritic shape of snow crystals was computed using a series of images photographed by W. A. Bentley. The specific images were dendritic crystals (P1d, P1e, P1f) and crystals that take a partial dendritic form and have ends or extensions (P2a, P2b, P2d, P2e, P2f, P2g) according to the Magono and Lee snow crystal classification. Image analysis, using known geometric relationships between length and width, and particle size distributions, examined the spatial properties of 50 sample snow crystals. Probability distribution functions were derived for SSA and these compared well with measured and other computed estimates of fresh snow SSA. For the non-rimed condition, the average SSA was 0.182 m 2 /g with a range from 0.09 to 0.33 m 2 /g. The presence of rime is discussed, and depending on the shape of the rime particles and the degree of surface coverage, the SSA can be doubled (20% coverage for needle or plate shaped rime). Fractal analysis was performed to determine various geometric relationships that characterize the dendritic form of snow crystal.

The MAGS Regional Climate Modeling System: CRCM-MAGS

The Mackenzie GEWEX Study (MAGS) regional climate modeling system (denoted CRCM-MAGS) is a developmental version of the Canadian Regional Climate Model (CRCM) tailored for use over North America. It is composed of three major components: the dynamical kernel of the CRCM, the operational physical parameterization package of the Canadian Centre for Climate Modelling and Analysis (CCCma) atmospheric general circulation model (GCMIII), and a high resolution land surface database developed for MAGS. In addition, surface runoff can be routed offline using the WATFLOW hydrologic model and a drainage database constructed over North America for the CRCMMAGS. Although closely related to the CRCM, the CRCM-MAGS differs in several major aspects. The Mackenzie Basin climate from a five year (plus 21 month spin up period) simulation was evaluated against surface observations. The model simulates a mean annual precipitation bias of about 13%, and a cold surface temperature bias of less than 1 ° C in both summer and winter. These results suggest that the GCMIII physics package, developed for a relatively coarse resolution GCM, can be used successfully in a high resolution regional climate model with minimal modification, provided a realistic surface representation is included.

Climate Impacts on Hydrological Variables in the Mackenzie River Basin

The research described in this paper examines changes in the hydrologic cycle in the Mackenzie River Basin (MRB) in northern Canada. The study focuses on temperature, precipitation, runoff, evapotranspiration and storage. A distributed hydrological model is used with two different climate input data sets: Environment Canada gridded observed data and the European Centre for Medium-range Weather Forecasting (ECMWF) reanalysis climate data (ERA-40). Both data sets were used to estimate runoff and evapotranspiration. The resulting hydrological variables were assessed for trends on a monthly and annual basis using the Mann-Kendall non-parametric trend test. The results reveal a general pattern of warming temperatures, and increasing precipitation and evapotranspiration. However, an overall decrease in runoff and in storage were detected for results derived from the Environment Canada data set while an overall increase in runoff and in storage were detected for results derived from the ECMWF data set. The sensitivity of mean annual runoff to changes in climate was also estimated using a non-parametric estimator. The results of the analysis can be used to better prepare for the potential impacts of climate change on water availability and water resource infrastructure in the MRB.