Hydrology in the landscape and digital elevation models Gianni Gorgoglione

Figure 1 Lines between ditches

Figure 2 Lines between wells Hr and H0

Figure 3 Water table section between two streams

Groundwater level estimation

Aim

On both dykes there are ditches draining the area. The average water level in the ditches as well as the elevation of the area is known. Nowadays the value of the area for human use is very low and it is mainly their value for natural protection that counts. Since the maintenance of the dikes is very expensive, the municipality is interested in the vegetation that might develop if the current use is abandoned. To estimate this a map of the groundwater level is required.

Methodology

The study area of Buschhagenniederung, Germany was projected with the national grid of DHDN_3_Degree_GK_Zone_3_E-N. For each ditch was created a layer on West and East side. The Western side had 4 observed heights and the Easter only 2 known points. The points on the ditches were interpolated by using IDW with variable rate. Afterwards, a set of points along the ditches were created to use as mask for the extraction of the interpolated values. In order to calculate the groundwater height (Hx) between the ditches (See figure 1), first it was necessary to add 7 meters and then to create lines needed to put the points every 20 meters (x). Thus, the calculated heights (Hx) were insert into these points for the water table lines (See figure 2 and 3).

HrHr

Ho

Hr Ho

Hydrology in the landscape and digital elevation models Gianni Gorgoglione

The formula used to estimate the height of the water table was the Darcy´s law referring to the case of an unconfined aquifer between two streams as follows:

The formula was applied on an Excel data sheet (See figure 4 below) The ground water flow rate (qn) (recharge) was given as 50 mm/annum and then converted in m/s.The hydraulic conductivity (K) was given as 100 cm/day and then converted in m/s The distance (R) between Hr and H0 was calculated in ArcGIS with Euclidian distance tool.A distance (x) of 20 meter was chosen between the points along the line.

Figure 4 Calculation of water table Height (Hx) between 2 streams

The next step was to insert the heights (Hx) into the points along the lines. The creation of these points over the study area made possible the interpolation by using IDW in order to estimate the groundwater table.

The result of the interpolation was later subtracted by 7 meters with the aim of calculating the ground water table above the sea level since the bedrock was 7 meters below the sea level.

Hydrology in the landscape and digital elevation models Gianni Gorgoglione

A second map to show the distance between land surface and ground water table was produced by using the following equation:

water-table elevation (W) = land-surface elevation (L) – depth to water below land surface (D)

then follows that

D= L-W

Result

The first result for the area of study was the water table heights in meters above the sea level (See figure 5) between the two streams. The lowest ground water level is located along the eastern side of the map

Figur 5 Ground water level above sea level

Hydrology in the landscape and digital elevation models Gianni Gorgoglione

The second map (figure 6) shows the depth to the water from land surface. This can give an idea of how the water table is distributed over the area. The green zones are those area of soil with major deep before finding the ground water beneath the land surface. The yellow and red zones are those areas where ground water is present at shorter distances from the land surface. From the data it possible to detect area where the Groundwater level is above the land surface, in fact the values of these area are below zero.

Figure 6 Depth to the Groundwater from land surface

Discussion

This basic approach within the limits of this exercise shows how it is possible to create the ground water estimation under the land surface with limited data. The depth to groundwater map shows that groundwater in some area is really close on the land surface. Some spot has ground water level at the same land surface level. Perhaps, this is not particular surprising because the nature of the land surface distribution. The interpolation could be better performed by trying different methods and techniques.