Classification of groundwater levels
The conduct of the groundwater level varies every time. This behaviour is indicated in groundwater steps. The groundwater level system has been established through observations over a series of years. The groundwater level is registered based on monitoring wells in which the groundwater level was measured at regular intervals. Groundwater levels are derived from data from this monitoring network. The average groundwater level (AGL) and the average lowest groundwater level (ALG) form the basis of a groundwater level (GL). With these GLs we indicate the behaviour of the groundwater in the soil.
Since 1966 we have divided them into seven levels, which are indicated in Roman numerals I to VII. Around 1977 a further subdivision was made of GLs III, V, and VII, in terms of the Average Highest Groundwater level. The designation * has been added, indicating that the AHG is in the range 20 to 40 c below ground level. For example, a III* indicates that the average highest groundwater level is 20 to 40 cm below ground level and the average lowest groundwater level is 80 to 120 cm below ground level. In popular parlance, this is called a “dry three”. After all, with an “ordinary three” the AHG is above the 40 cm ground level. This system is used to characterize the groundwater levels in the soil mapping.
Since 1988 the GL system has been expanded again. It appeared that specific groundwater level situations with the GLs used until 1988, were not distinguished separately. These were areas where there was either a very limited or a strongly fluctuating groundwater level. Through expanding the GL system, these characteristics can be indicated more clearly. The new GL system can be seen next to the old GL system in Table 1.
|Ia||< 25||< 50||I|
|Ic||> 25||< 50||I|
|II||-||50 - 80||II|
|IIa||< 25||50 - 80||II|
|IIb||25 - 40||50 - 80||II*|
|IIc||> 40||50 - 80||II*|
|III||< 40||80 - 120||III|
|IIIa||< 25||80 - 120||III|
|IIIb||25 - 40||80 - 120||III*|
|IV||> 40||80 - 120||IV|
|IVa||40 - 80||80 - 120||IV|
|IVc||> 80||80 - 120||IV|
|V||< 40||> 120||V|
|Va||< 25||> 120||V|
|Vao||< 25||120 - 180||V|
|Vad||< 25||> 180||V|
|Vb||25 - 40||> 120||V*|
|Vbo||25 - 40||120 - 180||V*|
|Vbd||25 - 40||> 180||V*|
|VI||40 - 80||> 120||VI|
|VIo||40 - 80||120 - 180||VI|
|VId||40 - 80||> 180||VI|
|VII||80 - 140||> 120||VII|
|VIIo||80 - 140||120 - 180||VII|
|VIId||80 - 140||120 - 180||VII|
|VIII||> 40||> 120||VII*|
|VIIIo||> 40||120 - 180||VII*|
|VIIId||> 40||> 180||VII*|
The Roman numeral and any letter behind it indicate how high or how deep the groundwater is; they provide quantitative information. A letter before the Roman numeral provides qualitative information about the special circumstances in which the soil is located.
These conditions are important in giving a good impression of the water balance. Qualitative additions are:
- B; Ground outside the dykes, periodically flooded (flood plains)
- S: False mirror. The level of the AHG is determined by periodically occurring groundwater levels above a poorly permeable layer (for example a boulder clay intermediate layer), under which an unsaturated zone occurs. Only for Soils with a groundwater fluctuation (ALG minus AHG) or more than 120 cm.
- W: Water above ground level for a consecutive period of more than one month during the winter. Only on land within the dike (for example nature reserves).
Due to the new groundwater level classification, specific groundwater level situations can be better represented on the soil map. The data collected in recent decades about groundwater behaviour can still be used to maintain the existing main classification.
Desired groundwater levels
For optimal grassland use, the groundwater should not be elevated for long periods. If this does happen, we speak of flooding. The average highest groundwater level must not decree 30 cm below ground level. For maize and arable plots, this is 50 cm. Groundwater level that is too low can be detrimental to damage that can be caused by drought. For various grounds, the existing GL is a fact that can hardly be changed. In higher sandy areas, the GLs VI, VII, and VIII often occur. A GL that has few restrictions on production and use is the GL IV. However, it is difficult to use on a larger scale in agriculture. Moreover, the soil type is quite dependent. Through dewatering measures and drainage, an attempt is made to achieve a GT LLLb and/or GLVb as much as possible. For maize and arable farming, a GT IV or VI is desired. The desired groundwater levels are the starting point for a ditch water level that has to be set. They also serve as the basis for calculating the ideal drain depth and drain distance. To achieve the desired groundwater levels, the ditch water level must be at least 80 cm below ground level for grassland and at least 100 cm for maize. This alone is usually not enough because the distance between the locks is usually too big. Therefore, additional drainage is often required, of which the depth must be at least 80 cm or 100 cm.