Integrated river catchment management - a network for
optimized water management, rehabilitation and
protection of aquatic ecosystems in Lake Lohjanjärvi
catchment area
Three dimensional (3D) modelling of Lake Lohjanjärvi
Inkala Arto and Lauri Hannu, Environmental Impact Assessment Centre of Finland Ltd
Description of the project
Lake Lohjanjärvi, the largest lake in Uusimaa province, is one of the most important
lakes for the recreation activities in the southern Finland. To keep the Lake Lohjanjärvi
attractive also in the future, it is important to know quantitatively the processes affecting
the water quality of the lake. Mathematical modelling of the lake and the lake catchment
area provide powerful method for investigation of the current state of the lake and for
forecasting the effect of different management scenarios.
A three-dimensional lake water quality and ecosystem model was applied to the lake, and
a distributed catchment model to watersheds of two main incoming rivers Nummenjoki
and Karstunjoki. The models were used to assess the impacts of the human activities in
the lake surroundings as well as simulate impacts of possible point load and distributed
load management scenarios, such as the effect of relocation of wastewater outlets and
waste water load reductions. Also an operational system for running the lake and river
load forecast models was developed. The system includes a www-interface for viewing
the model results and measurement data via the internet.
Technical solution
A hydrodynamic model with coupled water quality and algae model was used to model
the Lake Lohjanjärvi. The model uses a rectangular grid for which flow, nutrient
transport and algae behavior describing equations are solved. The model grid has a 70
meter horizontal grid resolution and ten vertical layers. Vertical resolution was variable
from of one meter near the surface to about 15 meters near the deepest bottoms. The
calculated variables were total nutrients (phosphorus and nitrogen), turbidity, suspended
solids, natrium, oxygen, biological and chemical oxygen demand, dissolved and detritus
forms of nutrients and the algal biomass. Lake model simulations were made for years
2002-2004, and were verified by comparing modeled results with the results from
Karjaanjoki Life measurement program and satellite images.
The above mentioned lake catchment areas were modeled using a distributed catchment
model. The model uses a grid-bases approach, in which the target catchment area is
divided into rectangular grid boxes, each having a land use, soil type, elevation and slope
data derived from national raster databases. Two resolutions, 200m and 400m were used
to model the target areas. Modeled variables were river flow, total nutrients (Ptot, Ntot),
soluble phosphorus (PO4) and suspended sediment.
Results and impacts
Three scenarios for relocation of the wastewater outlets and the effects of internal loads
were calculated. Results clarify phenomena affecting the lake water quality and clearly
show the relative effects of different nutrient sources, providing important information to
public and lake related decision making.
The catchment area model was used to simulate current state of the catchment areas.
Good agreement with discharge and acceptable agreement with modeled water quality
variables was obtained. Several load reduction scenarios were computed, including, for
example, addition of protective grass strips for agricultural areas.
Figure: Point load relocation scenario result, eastern part of lake Lohjanjärvi, change in
average wastewater concentration
Figure: Classification of grid boxes according to relative phosphate load, three year
computation