Different ecological processes will occur at different parts of the hydrograph. Although the pattern of the annual hydrograph is fairly predictable, its magnitude is not. Runoff routing is a procedure to calculate a surface runoff hydrograph from rainfall. The non-linear reservoir has "no" usable unit hydrograph. Drainage density can affect the shape of a river's hydrograph during a rain storm. The record of flow over time is called a hydrograph. Nevertheless, various graphical and empirical techniques have been developed to perform these hydrograph separations. Agriculture has changed the hydrograph, increasing peak runoff flows and shortening the length of runoff.