In previous reports SMED has used the hydrological model, HBV, for
calculation of runoff and its additional model, HBV-NP, for water quality,
to calculate the retention of nitrogen and phosphorus. In connection with
demands for finer geographical division SMED conducts a review of the
methodology for the various steps of calculation that is the basis for
HELCOM PLC Periodical reporting.
The HYPE model is in its structure adapted to handle a large number of
calculation areas. As the runoff data for future HELCOM PLC Periodical
reporting is expected to be set up for about 22,000 sub basins, a replacement
of the previously used HBV model will be necessary.
Before PLC6 calculations are carried out it is important to clarify the
impacts of a change in hydrological model, for calculating runoff and
ground leakage of nutrients, load and source distribution.
For analyzes based on results from future calculations to be compared with
previous analyzes of the previous reporting results in the periodic PLC
report it is important that the differences between the various models are
clarified.
A big difference from before is that runoff calculation using the HYPE
model is dependent on land use compared to the same calculation with the
HBV model. Runoff calculated in HYPE is dependent on land use which
means that the model calibration parameters are linked to a specific land
class, consisting of a combination of soil type and land use. The calculation
of leakage of nutrients, which is runoff-dependent, is consequently
implemented differently in the two models.
This project evaluates the differences in the calculation results that will
eventually be used in TBV (Technical calculation tool, water, Swedish
acronym). The model differences described in this project does not handle
differences due to the model set (eg S- HYPE and HBV -NP sets for PLC5).
It is not possible to evaluate the hydrology without regard to land use. The
report analyzes the differences in estimated runoff and discharge between
HBV and HYPE for areas with a dominant share of forest and open land.
Open land is defined in the same way as for the HBV model which is the
remaining land apart from forest.
The purpose of the theoretical evaluation is not to judge which of the
hydrological models is best. Both models have pros and cons, and both
models are currently used at SMHI. The purpose of this evaluation is to
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understand how a change in hydrological model will affect load calculations
of nutrients from land to sea.
The hydrological comparison has been divided into a number of different
parts. The models are compared on a regional scale and in smaller areas
with homogeneous land use. Further, a regional analysis of runoff is
evaluated in three different area sizes; smaller than 200 km2, between 200
and 2000 km2 and greater than 2000 km2. The regional comparison, with
runoff from HBV/PLC5 and S-HYPE is done in TBV to compare how
different expected runoff data affect the gross load of nitrogen and
phosphorus to the sea.
The evaluated modeled period is the same reference period used in PLC5,
1985-2004. The HYPE model simulates a slightly lower runoff than the
HBV model which leads to a lower gross load of total nitrogen and
phosphorus to the ocean.
In the comparison between the total runoff and soil specific runoff
differences regionally are quite small but as the geographical areas in future
reports will be conducted on a much finer scale, it can have effects locally.
When comparing the total runoff as used in TBV with the soil-specific
runoff, the primary differences appear in swamps, wetlands, urban land and
clearings. Calculations of retention in SMED HYPE will therefore take
place in a different gross leakage than the same calculation calculated in
TBV. In addition to differences in retention it means that when the total
runoff is used in TBV, the leakage of nutrients will be lower, due to the
lower drainage.
The differences between the predominant sources, open land, forest and
agriculture are relatively small. However, on a local scale, the differences
may become important for these land use types. The evaluation was only
done on yearly average runoff. How the differences over the year looks are
not investigated. TBV use monthly average runoff.
The difference in gross load from diffuse sources of nitrogen calculated in
connection with PLC5 calculation, about 132 000 tonnes / year, was reduced
by 3% when including the new estimated runoff Estimated runoff at a finer
geographical division reduced the load by 12%. Gross load from diffuse
sources of phosphorus were calculated in relation to PLC5 calculation to
3900 tons / year. When including the new estimated runoff, the load was
reduced by 2%. When runoff was calculated on a finer geographical
division, the load was reduced by 11%.
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The project findings suggest that differences in input between the
assessment base will dominate over differences in hydrological model.
Thus, no major differences between the models that will affect the runoff in
TBV have been noticed. Differences in retention due to changes in the
model of runoff calculations should be commented on in the context of the
work done in PLC6. The difference in runoff from agricultural land,
compared to the same weighted total runoff is relatively small. No
differences that need to be handled in any special way due to model change
in connection with PLC6 have been highlighted in this project.