There are different methods of hydraulic calculation of groundwater flow in a homogeneous medium with a flat sloping water table. These methods are characterized by different degree of approximation. Thus, the formulas of G. N. Kamensky do not have a strict theoretical justification. The method of Acad. N. N. Pavlovsky's method is also approximate to a certain extent, since according to this method of calculation the surfaces of equal heads in the conditions of a flat problem are considered as vertical planes. The method of N.N. Pavlovsky can be considered as the most accurate at present, because it is based on a strict mathematical conclusion and its main provisions are very close to the true picture of motion when the slope of the water table and the slope of the depression surface are small. The new method proposed in this paper is also an approximation, but it is based on a fairly rigorous mathematical derivation. Determination of flow rate and construction of depression curve by this method is simpler and more convenient than by the method of N. N. Pavlovsky.

Geographical position. Yan-shan ridge is located in the northern part of He-pe province of Northern China. It is located between 40-41° of north latitude and 115-120° of east longitude. This ridge separates the plateau of Inner Mongolia from the North China Plain. The Great Wall of China was built along it in ancient times. Geologic structure. The orogeny that resulted in the folding of ancient sediments in various areas of China during the Upper Jurassic and Cretaceous was called the Yanshan orogeny. No orogeny earlier than the Yanshan folding is known in North China, except for a putative orogeny of pre-Xinian time. The Sinyi system basically corresponds to Upper Proterozoic. time. Beginning in the Sinaian, down to the Jurassic, sedimentary rocks were accumulated in parallel horizontal layers. The layers in this stratum lie more often in accordance with each other, but sometimes stratigraphic disagreement is observed. The disagreement is most pronounced between Ordovician limestones and various rocks of the Middle Carboniferous. This indicates that North China was a landmass during the Silurian and Devonian periods.