Storm water is the term used to describe surface runoff generated by precipitation events. It includes water that flows over impervious surfaces like roads, rooftops, and parking lots, as well as pervious surfaces like fallow lands. Proper management of storm water is essential in agricultural and civil engineering to prevent soil erosion, manage drainage systems, and mitigate the risk of localized flooding.

Graded terraces are designed to manage excess runoff in high-rainfall areas. They function by diverting surplus water at a non-erosive velocity toward a safe outlet, such as a grassed waterway. In agricultural engineering, a threshold of approximately 80 cm of annual rainfall is generally considered the point where graded terraces become necessary to prevent significant soil erosion and waterlogging in cultivated fields.

The Probable Maximum Flood (PMF) is the most severe flood that is considered reasonably possible at a particular location based on the most extreme combinations of meteorological and hydrological factors. It is used primarily for the design and safety evaluation of high-hazard dams.

The critical exit gradient occurs when the seepage force exerted by water flowing through the soil becomes equal to the submerged unit weight of the soil particles. At this point, the soil loses its shear strength, leading to 'piping' or boiling, which can cause structural failure in hydraulic works like dams or weirs.

Khosla's theory of independent variables explains the seepage flow under hydraulic structures. It posits that the exit gradient is highest at the downstream end. Consequently, the process of undermining, or piping, begins at the downstream end where the water exits the soil, and it progresses backward toward the upstream side as the soil particles are washed away.

Direct runoff is generally defined as the water that enters stream channels immediately after a precipitation event. While the provided answer key suggests 'Sub-surface run off and deep percolation', standard hydrology defines it as surface runoff plus prompt interflow. This is an EXPLANATION_CONFLICT regarding standard hydrological definitions.

Pitching is the specific civil engineering term for placing stones or concrete blocks on the slope of an embankment or canal bank to protect it from the erosive action of water waves and currents. This is a standard practice in hydraulic structures to ensure long-term stability.

Free board is the vertical distance between the crest of an embankment and the maximum water surface level. It is specifically provided to prevent overtopping of the embankment due to wave action or unexpected surges in water level.

A hydrograph is a fundamental tool in hydrology that displays the discharge or flow rate of a river or stream over a specific period. It is essential for analyzing flood events, water resource management, and designing hydraulic structures. By observing the shape of the hydrograph, engineers can estimate peak flow, total runoff volume, and the timing of water movement through a catchment area.

Pitching refers to the practice of placing stones or concrete blocks on the slope of an embankment or canal bank to prevent erosion caused by water flow or wave action. While related to revetment, 'pitching' is the specific term for the placement of these protective materials.