*Inflow conditions*

The inflow from the north and west into the model area is governed by the transition zone between the mountain terrain and the plain where a high conductivity can be assumed. Here the inflow consists of the surface water run off from the mountains that depends on the precipitation rate. But due to investigations even after a number of dry years the total inflow did not disappear, but decreased from a long term mean value of about 0.7x109 m3 /a to 0.4x109 m3 /a could be observed by the Chinese partners. This amount of water could not only result from rainfall runoff from the mountainous domain. Therefore it was assumed a split of the horizontal groundwater recharge from groundwater inflow into the model area (see Fig. 4). One part is a rain-dependent contribution which in 'normal years', with a mean precipitation rate of about 590 mm/a is in the range of 0.4x109 m3 /a. This value is scaled in dependency of the mean precipitation rate of the current year. In a dry year with a precipita‐ tion rate of 380 mm/a, for instance, the rain dependent inflow to 0.26x109 m3 /a. For this con‐ tribution the imagination is that a part of the precipitation of the mountain slopes infiltrate on the surface and percolate down to the bedrock basis. On the relatively impermeable bed‐ rock the water flows as shallow groundwater aquifer in the loose stratum with low thick‐ ness into the model area. The loose stratum depositions in the piedmont plains consist of boulder, gravels and sand with inclusions of local loess and loessloam lenses. These loose stratum depositions are well permeable and this inflow contribution from the mountains is asssumed to be directly dependent on the precipitation.

**Figure 4.** The groundwater inflow regime into the NCP

confined and unconfined aquifers. Because of the multi-layered geological structure of the loose stratum in the model area, consisting of loess, alluvial loess, sand-gravel-cobbleboulder sediments with more or less mighty clay and silt inclusions, the details of hydro‐ geological conditions are complicated. Therefore the used spatial distribution of the hydraulic conductivity and specific yield (both important for good model results) are ad‐ equate phenomenological descriptions of mean values and not a detailed representation

The aquifer characteristics were determined mainly on the base of the interpretation and evaluation of the above mentioned borehole data. The borehole data represent the geologi‐ cal layers (boring logs) at single points. They show high variations from one point to anoth‐

**1.** In a first step the local single point data have to be transformed in values representative for an associated area (meso-scale values) [11]. For this step the information and data from thematic maps (e.g. Beijing hydrogeological maps lithology map, water abun‐ dance map, etc.) were included. The validity range of these meso-scale values could be specified with the informations from a water abundance map representing the water yield and water storage capability. These data resulted primarily on measurements in

**2.** In a second step the discrete meso-scale values were interpolated within the model area

On the basis of the derived values a fine tuning was realized in order to get minimal differ‐

In [12] a mean storativity in the range of 0.08 and 0.18 has been estimated. Due to hydrogeo‐ logical investigations of borehole data a regionalisation of the hydrogeological parameters could be performed, yielding a mean storativity of 0.13. The values of S0 changes from 0.19

The inflow from the north and west into the model area is governed by the transition zone between the mountain terrain and the plain where a high conductivity can be assumed. Here the inflow consists of the surface water run off from the mountains that depends on the precipitation rate. But due to investigations even after a number of dry years the total

 -values of the aquifers range from 2.0x10-3m/s in the region of the piedmont plains and the alluvial fan plains to 0.1x10-4m/s in the flood plains. The loose stratum depositions can be classified according to German Institute for Standardization Guideline 18130 as per‐

water exploitation wells. With this approach meso-scale values (for kf

**3.** In a third step the absolute values of the smoothed spatial distribution of kf

and S0 were deduced from these borehole



and S0 were

of local conditions in reality.

30 Water Supply System Analysis - Selected Topics

The kf

meable to very permeable.

*Inflow conditions*

data due to the following procedure:

er. In particular the hydrogeological parameters kf

spatial distribution could be determined.

adapted to fulfil the water budget of the model area.

ences between calculated and measured groundwater surface map.

(piedmont plains) to 0.03 (flood plains). This spatial distribution of kf

also a basis for the regionalisation of the inflow boundary conditions.

The second contribution to the horizontal groundwater recharge from the mountainous area is of about 0.3x109 m3 /a and it was implemented deeper. Here the underlying idea is that this component corresponds to the part of the precipitation which infiltrates in the mountainous area through clefts into the deeper rock formations. The groundwater flow system in the bedrock consists of macroscopic structures and cavities linked with each other, as for exam‐ ple fracture networks, faults, layer joints, dissolution features and conduits etc. The ground‐ water inflow depths were assumed according to the distribution of the water bearing carbonate rocks, sandstones and crystalline rocks in the hydrogeological map of Beijing. These so-called deep inflows are not dependent directly on the precipitation and change on‐ ly in terms decades.

The total horizontal groundwater recharge (inflow) ranges from 0.55 to 0.75x109 m3 /a. The quantitative split is to some extend arbitrary and based only on plausibility considerations since none of the components can be measured directly.
