**The subsurface runoff is also known as:**

**A. Interflow**

Interflow refers to the movement of water within the soil layers above the water table. It occurs after precipitation, when the excess water infiltrates the soil and moves horizontally through the unsaturated zone. Interflow is an important component of subsurface runoff as it contributes to the overall flow of water through the watershed.

**B. Storm seepage**

Storm seepage refers to the movement of water through the soil during and immediately after a storm event. It occurs when the rainfall exceeds the infiltration capacity of the soil, causing water to flow overland. Some of this water may also seep into the soil, contributing to subsurface runoff.

**C. Secondary base flow**

Secondary base flow refers to the flow of water in streams and rivers that is derived from subsurface sources. It is called secondary because it occurs after the initial surface runoff has subsided. As the water infiltrates the ground, it may percolate downward and eventually reach the streams through subsurface pathways, contributing to the base flow.

**D. All of the above?**

Yes, subsurface runoff encompasses all the above-mentioned terms. It includes the movement of water through the soil layers (interflow), the seepage of water into the soil during and after storms (storm seepage), and the flow of water in streams derived from subsurface sources (secondary base flow). These processes are interconnected and collectively contribute to the overall movement of water within a watershed.

**In summary, subsurface runoff, also known as interflow, storm seepage, and secondary base flow, represents the movement of water through the soil layers, the seepage of water into the soil during and after storms, and the flow of water in streams derived from subsurface sources. These processes are important components of the hydrological cycle and play a crucial role in the overall water balance of a watershed.**