Understanding Hydrological Systems and Watersheds

Hydrological System

Hydrological phenomena are extremely complex and cannot be known completely. However, we can represent them in a simplified form using the concept of a system, which is a set of interacting parts as a whole. The hydrological cycle is a system with components like precipitation, evaporation, and runoff. These components can be grouped into subsystems, allowing us to analyze them separately and combine the results based on their interactions.

Hydrological Model

The objective of hydrological systems analysis is to study the system's operation and predict its output. A hydrological model approximates the real system. Its inputs and outputs are measurable hydrologic variables, and its structure is a set of equations or transfer functions that transform input variables into output variables. Hydrological models are classified into two categories: Physical Modeling and Mathematical Models. Physical models represent the system on a reduced scale, like hydraulic models. Mathematical models represent the system through functions relating output variables to input variables. Most hydrological processes are random and vary with time and space, making model development challenging and requiring simplification.

The Water Balance

Consider a system with an inclined, waterproof surface with an outlet at point A. When rain enters the system, surface flow develops at A. The water balance equation is: I - O = dS / dt (1.1), where I is the input per unit of time, O is the output per unit of time, and dS / dt is the change in storage within the system per unit of time. A minimum water height is needed for surface runoff to occur. Once the rain stops, the retained water continues to flow as residual flow. In reality, the water balance in a watershed is more complex, with various losses. Evaporation occurs from the start of precipitation. Water is stored in ground depressions, evaporates, infiltrates, or becomes runoff. The general water balance in a watershed can be visualized in Figure 1.7 and represented by:

a) Surface Water Balance: P - R + R_g - E_s - T_s - I = S_s (1.2)

b) Subsurface Water Balance: I + G₁ - G₂ - R_g - E_g - T_g = S_g (1.3)

c) Catchment Area Water Balance: P - R - (E_s + E_g) - (T_s + T_g) - (G₂ - G₁) = (S_s + S_g) (1.4)

Watershed

General

Watersheds, or drainage basins, are important hydrological regions for applying the water balance concept. A watershed is the area drained by a waterway or stream system, where waters converge to a point. It contributes to runoff and provides flow to the main channel and tributaries. Understanding a watershed's physical and functional characteristics is crucial for:

• a) Comparing watersheds
• b) Interpreting past events
• c) Forecasting river discharge

These characteristics depend on physical factors (tisiographic factors) and climate-dependent factors.

Physical Characteristics of the Basin

Basin Boundary

A basin is bounded by a line of highest topographic points, called the boundary, which divides precipitation between neighboring basins. The boundary is a rigid line around the basin, crossing the stream only at the outlet. The basin's flow includes surface runoff and subsurface water. Watersheds are delimited by topographic and groundwater divides. The topographic divide is determined by topography and defines the surface water area. The groundwater divide is determined by geological structure and influences base flow. Due to the difficulty in determining the exact groundwater divide, the topographic divide is commonly used.

Basin Area

The basin area is the flat area within the boundary. It is a basic element for calculating other physical characteristics and is usually determined with a planimeter, expressed in km² or hectares. Watersheds with the same area can have different behaviors due to other factors.

Basin Shape

The basin's shape affects the time of concentration, which is the time required for the entire basin to contribute to flow at a specific section. Large basins often have a pear shape, while small basins vary depending on their geological structure. Indices like compactness ratio and form factor are used to relate basin shapes to known geometric shapes.