Critical flow occurs when the specific energy of the flow is at its minimum for a constant discharge. At this state, the Froude number is equal to one. This condition is a fundamental concept in open channel hydraulics, marking the transition between sub-critical and super-critical flow regimes.

The density of water is approximately 1000 kg/m³ at 4°C, which is the temperature at which water reaches its maximum density. While the option provided uses 'ka' instead of 'kg', it represents the standard scientific value used in agricultural engineering and fluid mechanics calculations for water density under these specific environmental conditions.

According to Newton's law of viscosity, the shear stress in a Newtonian fluid is directly proportional to the velocity gradient or the rate of shear strain. The constant of proportionality is the dynamic viscosity of the fluid, which remains constant regardless of the applied shear rate.

An influent stream is a stream that loses water to the groundwater system, whereas an effluent stream is one that gains water from the groundwater system. There is a potential conflict in terminology here as standard hydrogeology often defines an effluent stream as one gaining water from the water table. The provided answer 'Influent' may be context-dependent or potentially mislabeled in the source material.

In a pipe flowing full under steady conditions, the velocity distribution is parabolic due to friction at the pipe walls. The fluid particles near the center of the pipe experience the least resistance from the walls, resulting in the maximum velocity. Conversely, the velocity is zero at the pipe walls due to the no-slip condition, gradually increasing towards the center.

The discharge (Q) over a rectangular notch is calculated using the formula Q = (2/3) * Cd * L * sqrt(2g) * H^(3/2), where Cd is the coefficient of discharge, L is the length of the notch, g is the acceleration due to gravity, and H is the head of water above the sill. This formula is fundamental in hydraulic engineering for measuring flow rates in open channels.

A Venturi meter is a flow measurement device designed to minimize head loss, resulting in a very high coefficient of discharge, typically ranging from 0.95 to 0.99. In contrast, weirs and orifices have significantly lower coefficients due to greater energy dissipation and flow contraction effects.

A rod float is a weighted vertical rod designed to float upright in a stream. By measuring the time it takes for the rod to travel a known distance, one can estimate the mean velocity of the water column. While current meters are more precise, rod floats are a traditional method for directly estimating depth-averaged velocity.

A piezometer is a simple device consisting of a tube inserted into a pipe or container to measure the pressure of a liquid. It measures the pressure head by the height to which the liquid rises in the tube. Since the top of the tube is open to the atmosphere, the pressure measured relative to the atmospheric pressure is known as gauge pressure. It is not suitable for measuring vacuum or absolute pressure directly.

Cavitation occurs when the local static pressure of a liquid drops below its vapor pressure at the prevailing temperature. This causes the liquid to vaporize, forming small bubbles or cavities. When these bubbles move into regions of higher pressure, they collapse violently, creating shock waves that can erode solid surfaces like pump impellers or turbine blades. Therefore, low pressure is the fundamental trigger for the initiation of the cavitation process.