Lake water levels for calibration of the S-HYPE model

Sweden has about 100,000 lakes with a surface area >0.01 km². Water levels in the six largest lakes are measured and reported daily on the internet by the Swedish Meteorological and Hydrological Institute (SMHI). In addition, water levels in natural lakes are measured with the main purpose of estimating discharge through established rating curves. The raw water level data are usually not used for other purposes. Water level data are, for instance, not quality controlled and corrected, for example for ice jams, in the same way as discharge data. Lakes have a major impact on the hydrological and hydro-chemical conditions, and lake routing has been modelled explicitly in the Swedish HBV model since 1985 (Bergström et al. 1985).

S-HYPE is a hydrological model that covers all of Sweden in high spatial resolution (Strömqvist et al. 2012). It is based on the HYPE model code (Lindström et al. 2010). Both HYPE and S-HYPE are developed continuously and current versions are available at http://hype.sourceforge.net/ and http://vattenwebb.smhi.se/. The present S-HYPE version (2012 2.0.0) has 36,692 sub-basins with a median area of 7 km². Lake water levels are explicitly modelled for 8,581 lakes. There are about 400 discharge stations in the model, which means that about 99% of the basins are ungauged. It is practically impossible to measure discharge at all points of interest, which is why models are used as a complement. A large number of discharge measurements are required, at different flow situations, before a reliable rating curve can be established. The cost of establishing a rating curve based on ten measurements was estimated at roughly 17,000 euro (Lena Brahm-Eriksson, personal communication). High flows, in particular, are difficult to capture and measure. Rating curves are thus often extrapolated for high flow situations. Rating curves at the SMHI are divided into 1, 2 or 3 intervals, corresponding to 3, 7 or 11 parameters. The large number of parameters makes it difficult to generalize this information to the many ungauged lakes in S-HYPE.

Since 2008, the SMHI has operated a network of temporary water level measurements that are rotated to new locations after 2–3 years. The idea is that even short time series can provide valuable information, especially on the dynamics of a catchment (see also e.g., Seibert & Beven 2009). The objectives of this study were to:

1. improve the lake routing scheme of the HYPE model;

2. test if lake water level data are useful for calibration of hydrological models even without measuring discharge;

3. test if a hydrological model combined with lake water level data could be used in the establishment of rating curves as a cost-effective complement to existing techniques.

The S-HYPE model was originally calibrated with general parameters, i.e., parameters that were constant in the whole application or coupled to soil type or vegetation type (Strömqvist et al. 2012). These parameters were mainly calibrated for small, lake-free representative basins, dominated by specific land-uses and soil types. General lake routing parameters were thereafter established for a set of small basins. It proved difficult to find parameter values that were applicable to all of Sweden. For instance, the same characteristics for clay were assumed in the southern part of the country as in the north. To allow adaption to local conditions and improve model performance, the concept of super parameters has been introduced in the HYPE model. Super parameters have been formulated for both water quantity parameters and parameters which only affect water quality. They act within specified parameter regions and govern how key parameters deviate from the general values. Super parameters either affect only one parameter, or a set of related parameters. The following super parameters were used in this study:

1. Precipitation factor (%)

2. Evaporation factor (%)

3. Temperature correction (° C)

4. Recession coefficient factor for all soil layers and surface runoff (%)

5. Lake outflow factor (%).

The super parameters are a convenient compromise between a completely general model and a model where all parameters are calibrated for each gauging station. For instance, the relation between parameters and recession rates from individual soil layers can be maintained. This reduces the risk of unrealistic parameter combinations. The use of super parameters has improved the S-HYPE results considerably, and made the model more useful for forecasts and warnings, for example. The long-term vision is to continuously improve the model structure and input data and reduce the use of local adjustments.

The inverted value (1/c) gives a linearized time constant (T), which governs the time with which an inflow hydrograph will be delayed while passing through a lake. It is usually more convenient to convert T from seconds to days or hours. The time constant will vary with water level due to the non-linearity of the rating equation.

This thus suggests that p ≈ 2 is appropriate for parabolic sections. This shape was assumed to be more typical for natural lake outlets than either rectangular or triangular (with 90° angle) cross-sections, corresponding to p = 1.5 and p = 2.5, respectively (see further e.g., Maidment 1992).

This formulation can yield inaccurate results for water levels, due to numerical problems. The threshold w₀ may be several hundred metres above sea level, but the variations in lake water levels are only a few metres. It is, therefore, in practice, better to evaluate water levels above the threshold (i.e., h = w – w₀ instead of w).

Seven types of simulations were made (Table 2). The first (method Ref) is the reference to which the other tests are compared. It is the general S-HYPE model described above. Some of the eight test basins have been used in the general model calibration, but with very little influence from each one. In the reference situation, there is no calibration to local data using super parameters. The general lake rating parameters are used. In the second (4obs) and third simulations (max–min), the value of only a few observations was studied, both based on 2 years of data. In the second simulation only four water level observations were used, and in the third simulation only the range (max–min) was used. The four measurements in method 4obs were chosen as one measurement, the 15th of the months with highest and lowest recorded water levels for each lake, during the 2 years. These months are strictly speaking not known, but the hydrological regime in different parts of the country is well known. In the fourth method (kW), 10 years of daily water levels were used for calibration of the k parameter. In methods 2–4, only the rating parameter k was calibrated and the inflow was not modified.

In the fifth method (CalW), both the lake routing (k) and super parameters that affect the lake inflow were calibrated. This corresponds to a normal calibration but using only water levels instead of discharge. The official discharge was not used in the calibrations 2–5, but only for subsequent evaluations. The sixth method (CalQ) is the traditional situation in which the official rating curve (although simplified, see above) is used for the lake routing, and the inflow to the lake is calibrated using 10 years of discharge data (by the official rating curve). The last method (Q(W)) basically measures how well the rating curve established using only water level data corresponds to the official rating curve. This is done by comparing the discharges according to the two curves, both driven by the observed water levels (Wrec in Figure 1).

Out of 42 lakes for which simplified rating curves were developed, most had exponent p values of around 2. The median value was 2.0 and the mean 2.3, due to some lakes with high p values. For all lakes, a reasonable agreement could furthermore be established for p = 2, by adjusting the k value and w₀ appropriately. This is in agreement with the assumption of parabolic lake outlets made above, as well as values reported by Maidment (1992). The assumption of p = 2 as a general value thus seems appropriate.

A rough estimate of the range between minimum and maximum water levels can be made without actually performing measurements, for instance, from local knowledge or other soft data (see e.g., Seibert & McDonell 2002). It would, for instance, be very difficult to make a reliable estimation of the range in outflow from Lake Torneträsk during 2 years, 1999–2000 (=310 m³/s), without measurements (see Figure 3). It would, on the other hand, be much easier to make a reasonable estimate of the range in water levels (=1.85 m). Even a rough estimate improves the results considerably in many cases.

Full use of the water level data during 10 years, including re-calibration of the inflow, gave nearly as good discharge simulations (on average 0.917, Table 4) as a traditional calibration using recorded discharge (0.929). The water level calibration furthermore resulted in a slightly better simulation of water levels (0.932 versus 0.928). The largest part of the improvement is, however, already obtained by adjusting only the rating parameter k. This is easily done in practice. The adjustment of k improves the modelled water levels in the particular lake. If the lake is small compared to the upstream area (as for instance Kukkasjärvi, Räktjärv and Hassela), the downstream flow is not improved so much by this adjustment. However, for large lakes, such as Torneträsk in Figure 3, the downstream discharge is improved dramatically by only adjusting the rating parameter k. The use of water levels can improve the timing of discharge, by adjusting parameters affecting storage of water in a basin and the timing of snow melt.

In Table 3 it can also be seen that water level variations were modelled successfully after calibration. This is also the case for Lake Räktjärv, which is small compared to the upstream area (0.1%), and one of the lakes where the previous lake routing method failed completely. The reason for failure is that the time constant (T) at high water levels in this lake is only 6–7 hours, which is why a daily time step is not sufficient. The water levels were modelled adequately by the new lake routing scheme, both in the lakes in this paper and in other lakes in S-HYPE. This shows that the new lake routine works well in practice.

Volume errors were rather low to start with. The average volume error for the general model was −3%, and the average absolute volume error was 6% (Table 4). Only one of the calibrations using water level data (CalW) involved adjustment of the inflow volume. This calibration only slightly improved the volume errors compared to the general model. Full calibration using discharge reduced the average absolute volume error to 2%. NSE is much more sensitive to timing errors than to volume errors. In practice, even after a traditional optimization of NSE, there is often a significant remaining volume error (see e.g., Lindström 1997). The volume is, furthermore, quite uncertain in recorded discharge series. In practice, the mean discharge can change by ±5–10%, or even more, when a rating curve is revised. For flood warnings, the timing can be more important than the actual discharge level, since the modelled flow is usually compared to statistics based on model results from a reference period (see e.g., Bergstrand et al. 2014).

Water levels are easy and cheap to measure. They contain much of the information in a discharge record. Measured water levels were found to be useful for calibration of the S-HYPE model, even without measuring discharge and establishing a rating curve. Modelled water levels in the lake itself improve substantially, and on many occasions the water levels themselves are of great interest. Furthermore, the downstream flow is usually improved considerably if the lake is large in relation to the upstream area. It is mainly the timing of discharge, rather than the runoff volume, that is improved. What is usually referred to as recorded discharge in the calibration of hydrological models is actually modelled using a rating curve and recorded water levels as input. An alternative is to use the actual measurements, i.e., water levels, in the calibration. Extrapolation of the rating curve is thereby avoided. Water levels are furthermore less skewed than discharge. Water level data deserve more attention than they have so far received and more efforts should also be spent on quality control of these measurements. The main conclusions from the present study are as follows:

• A semi-analytical lake routing method was developed for the HYPE model in order to resolve fast variations in water levels in small lakes with large inflow. It was found that the exponent p in the rating curve equation could be set = 2 as a standard value.

• Hydrological models can be improved considerably, compared to a model with general parameters, by calibration to water levels, even without using measurements of discharge and establishing a rating curve. In the relatively large lakes in this study, the results were improved already by using only four observed water levels (mean NSE = 0.88 for both water levels and discharge), compared to the general model (−1.44 and 0.76, respectively).

• Calibration of both the rating curve of a lake, and the inflow from upstream areas, gave nearly as good discharge simulations (mean NSE = 0.92) as a more traditional calibration where recorded discharge data were used (0.93). Approximate rating curves can be estimated by model calibration and thereafter used with recorded water levels as input for calculation of a discharge. This approximation of the recorded discharge agrees well with the official recorded discharge.

The methods and results should be valid for other regions with lakes, for instance, the Nordic countries. In Finland, water levels are, in fact, already used for model calibration (Vehviläinen et al. 2005). Further work should be done on how hydrological models can be combined with water level data to establish both timing and volume of discharge, as a complement to current techniques. The results presented here, however, show that even very little information on water levels is useful for improved calibration of hydrological models, even without discharge measurements and traditional establishment of a rating curve.

This work was part of the SMHI contribution to the implementation of the EU Water Framework Directive in Sweden and was financed by the Swedish Government. Special thanks are due to the observational hydrology department at SMHI. The constructive comments from two anonymous reviewers are gratefully acknowledged.