Hydrological Definition of Time of Concentration ($t_c$):

• Definition: Time of concentration ($t_c$) is defined in surface hydrology as the time required for storm runoff to travel from the hydraulically most remote point of the watershed drainage basin to the basin outlet or design gauging point.
• Significance in Peak Runoff Design: When rainfall duration equals or exceeds $t_c$, the entire watershed area contributes simultaneously to the outflow, resulting in peak discharge ($Q_p$) in rational formula calculations ($Q = C I A / 360$).

Performance of Plotting Position Formulas for Small Hydrological Samples ($N \lt 25$):

• Plotting Formulas:
• California method: $P = m / N$
• Foster method: $P = (2m - 1) / (2N)$
• Weibull method: $P = m / (N + 1)$
• Official Key Context: In empirical rainfall frequency analysis comparative studies conducted for Indian semi-arid watershed data, when the record length is very small ($N \lt 25$), error minimization benchmarks rank the performance order as California > Foster > Weibull.

Hydrologic Abstractions in Rainfall-Runoff Relationships:

• Definitions:
• Total Rainfall Hyetograph: Plot of gross rainfall intensity or depth against time.
• Effective / Excess Rainfall Hyetograph (ERH): That portion of rainfall that becomes direct surface runoff.
• Hydrological Abstractions: The difference between total rainfall and effective rainfall represents the total hydrological abstraction (losses):
  $$\text{Abstraction} = \text{Total Rainfall} - \text{Excess Rainfall}$$
  Abstractions comprise interception by vegetation, surface depression storage, evaporation, and infiltration into the soil profile.