What is the primary condition that leads to runoff?

The primary condition that leads to runoff is when the infiltration capacity of the soil is exceeded. Infiltration capacity refers to the ability of the soil to absorb water. When precipitation occurs, if the rate of rainfall surpasses the soil's ability to absorb that water, it results in excess water that cannot be infiltrated into the ground. This excess water then flows over the surface, creating runoff.

The other options, while they may contribute to water movement or soil conditions, do not primarily result in runoff. Established plant roots can enhance absorption and reduce runoff, while high soil temperatures might affect evaporation and soil moisture but do not directly cause runoff. Compacted soil can reduce infiltration capacity, which may lead to runoff, but the key condition is the point at which the infiltration is overwhelmed. Thus, exceeding the infiltration capacity is specifically the condition that initiates runoff.