An MS Excel-based water-balance tool was used to fulfil these objectives.
A deterministic conceptual lumped MS Excel-based water-balance estimation tool was constructed to evaluate possible causes and mechanisms of the known but inconsistently documented water-level changes.
It should be noted that according to the used approach, all the errors from the estimation of other water-balance components are lumped together in the term [G.sub.o].
In order to maintain a reasonable ease of applicability of the tool and because of the occasional lack of sufficient data, the following assumptions had to be made in the compilation stage of the water-balance tool:
The water-balance tool was calibrated against the volume of the lakes corresponding to the GIS-based water volumes in May 2009, which, due to the data deficiency, was assumed to be the average and equilibrium lake level for the month of May.
The share of catchment runoff in the annual water-balance inputs to L.
Altogether 100 different yearly series were derived and used to run the water-balance tool 100 times from an empty lake to equilibration.
Therefore the question was: 'Could a significant change in catchment vegetation cause the lake-level changes?' Because meteorological data from the 1950s were missing, the effect of the changing composition of catchment vegetation had to be evaluated with the water-balance tool using a modern analogue.
The results indicate a possibility that one of the underlying assumptions of the water-balance tool surface--water and groundwater catchments having the same area--is not met and the groundwater component of the tool should be reworked from that perspective.
The specific research objectives were evaluated with the help of a spreadsheet-based water-balance simulation tool and the following conclusions were drawn.
In the case of the studied lakes a shift towards moister climate increased the water-balance input components and decreased water-balance output components between 1973 and 2009, causing a higher water inflow to the lakes.
A Monthly Water-Balance Model Driven by a Graphical User Interface.