(3) _The topography of the surface_ has much to do with determining the proportion of rainfall which becomes ground-water. If the surface be flat, more will sink in; if it be sloping, more of it will run off before it has time to sink. Other things being equal, the steeper the slope the larger the proportion of the rainfall which will run off over it. (4) _The texture of the soil_, or other material on which the rain falls, helps to determine what proportion of it sinks beneath the surface. If the surface materials be porous, the water sinks readily; if of close texture, it finds less ready ingress. Other things being equal, the closer the texture of the soil the less the proportion of the rainfall which will enter it. (5) _The texture and structure of the rock_ beneath the surface have some influence on the amount of ground-water. The rock may be stratified or massive; it may be abundantly or sparsely jointed; it may be porous or compact. On the whole, stratified rock is more favorable for the entrance of water than unstratified, partly because of its greater average porosity, and partly because the planes of division between beds often allow the passage of water. If the beds of stratified rock are vertical or inclined, water finds its way into them more readily than if they are horizontal, in so far as it descends along stratification planes. Horizontally bedded rock, or rock which is not bedded at all, may be so much jointed, and the joints so open, as to allow the water to enter readily. The conditions favorable to the sinking of abundant water below the surface are therefore heavy precipitation, falling slowly on a surface with little relief, a soil of open texture underlain by rock which is porous, or affected by vertical or highly inclined planes of cleavage. The annual discharge of water by rivers is estimated to be about 22 percent. of the rainfall on the land.[93] =Supply of ground-water not altogether dependent on local rainfall.=—The amount of ground-water in a given region is not always entirely dependent on the local rainfall. Ground-water is in constant movement, and entering the soil or rock at one point it may, after a long subterranean journey, reach a point far from that at which it entered. Thus beneath the Great Plains of the West there is much subterranean water which fell on the eastern slopes of the mountains to the west. It has flowed beneath the surface to the Plains, where some of it is now withdrawn for the purposes of irrigation in regions where rainfall is deficient. The accompanying diagram (Fig. 199) illustrates the flow here described. [Illustration: +Fig.+ 199.—Diagram illustrating the general point that ground-water is not dependent entirely upon local supply.] [Illustration: +Fig.+ 200.—Diagram illustrating the position of the ground-water surface (the dotted line) in a region of undulatory topography.]