1. What is deep percolation in the context of L'vovich's water balance? What is its value as a fraction of precipitation? Why? Explain the physical basis for this behavior.
Deep percolation is the fraction of precipitation that reaches the groundwater but does not return eventually to the surface waters as baseflow. Its ultimate destination is the oceans. Its global value is less than 5%, with an average of less than 2%. The value is a small fraction of precipitation because the predominant direction of groundwater flow follows a nearly horizontal gradient, to exit to the local/regional baseflow of nearby streams and rivers.

2. Contrast the contributions of Dalton, Penman, Monteith and Shuttleworth-Wallace in evaporation modeling. Use a tabular form for the comparison.

Dalton Penman Monteith Shuttleworth-Wallace

Mass-transfer method only. Combination method (mass transfer and energy balance). Physically-based combination method, featuring two resistances, external and internal. Physically-based combination method, featuring five resistances, accounting for plant canopy evaporation and the soil or water in which they are growing.

3. Explain the controversy regarding how to account for initial abstraction in the runoff curve number method, as interpreted by hydrologic models in current use, such as HEC-HMS and EPA SWMM. How did Mockus propose to solve the problem of quantifying the initial abstraction?
Mockus proposed to plot runoff vs effective precipitation, but was overruled by his superiors. They zeroed on the initial abstraction as a constant fraction of precipitation, with the constant set at λ = 0.2 from the beginning (1954). Initially, the runoff curve number method was intended to be lumped in time. i.e., for precipitation not to vary within the storm duration. However, in practice, users have specified the initial abstraction at the beginning of the storm duration, effectively rendering it distributed in time. Currently, there is no standard on how to temporally account for initial abstraction.
4. Why is the Mockus approach to runoff generation, for the most part, better than Horton's? Explain.
Mockus approach considers a finite depth of soil; on the other hand, Horton's approach assumes an infinite soil depth. Most locations have a finite soil depth; thus, Mockus' apprcach works better in most practical situations.
5. Why is the time of concentration calculated by the kinematic wave model likely to be much less than that calculated with a storage concept or diffusion wave? Explain the physical (mechanical) reason for this behavior.
Because the kinematic wave lacks diffusion and, consequently, it does not spread (reduce) the flows. Conversely, the diffusion wave has diffusion and, consequently, it spreads the flows in time and space. The theoretical kinematic time of concentration is exactly half of the theoretical time of concentration of the storage concept, which has built-in diffusion.
6. (a) What is the mean albedo of the Earth's surface? (b) Has it changed from ancient times to the present time? How much, according to published studies? (c) How does albedo vary from rain forests to deserts? (d) Is it possible for humans to change the albedo of the ground surface? How?
(a) 0.154; (b) Yes, it has increased, from 0.14 about 6000 years ago, to about 0.154 at the present time; (c) Rainforests, 0.07-0.15; deserts, 0.20-0.45; (d) Yes, by land use changes from forest, to range, to agriculture, to urban; generally, by an anthropogenic decrease in environmental moisture, in either soil or air.

7. Discuss the apparent reason for the persistence of the Sahel drought of 1970-85. What is the perceived role of transhumance (or the lack of it) in the Sahel drought?
Prior to 1970, persistent droughts were infrequent in the Sahel, lasting mostly one or two years. However, the drought that started in 1970 lasted 20 years. It appears that the transition from transhumance (nomadic propulations moving from north to south, and viceversa, to follow seasonal moisture) to sedentary pastoralism led to overgrazing and, consequently, to the widespread deterioration of the native semiarid decosystems, triggering a climatic change to a drier, more vulnerable, climate.

8. Why is the diffusion wave model of catchment dynamics likely to be more accurate that the kinematic wave models of HEC-HMS and Li? What is the reason for their different numerical behavior?
The diffusion wave model of catchment dynamics is a two-parameter routing model (Seddon's velocity and Hayami diffusivity), so its calculation of wave diffusion is second-order accurate. The kinematic wave models have only one parameter (Seddon velocity); therefore, they are saddled with inaccuracies due to their featuring of uncontrolled numerical diffusion. Thus, the diffusion wave model is shown to be grid independent, a condition which may be shown to be patently lacking in its kinematic counterpart.

9. What is the Walker cell? What is the root cause of the El Niño phenomenon? How often does an El Niño event likely to recur? What are the effects of an El Niño event?
The Walker cell is an atmospheric circulation cell or pattern residing along the equator. The root cause of the El Nino phenomenon is attributed to oceanic-atmospheric interactions along the Equatorial Pacific which result in the warm waters extending from west to east, to reach the coast of South America, near northern Peru. Strong El Niño events occur every 8 to 15 years. El Niño usually causes great floods, often paired with severe droughts in adjacent regions.

10. How much is the mean annual global terrestrial precipitation, in mm? How was this value obtained (calculated)? For what part of the climatic spectrum of precipitation are droughts likely to be more persistent? Why? Give an example.

The mean annual global terrestrial precipitation has been estimated at 800 mm. The mean atmospheric moisture is estimated at 25 mm. The atmospheric moisture recycles every 11 days on the average, constituting a total of 33 cycles per year, which amounts to 25 × 33 = 825 mm/yr, which may be rounded to 800 mm/yr. Droughts are likely to be more persistent around the middle of the climatic precipitation spectrum, for example, in the state of Idaho. In hyperarid regions, the drought events are shorter due to the relatively high drought frequency, Likewise, in hyperhumid regions, the drought events are generally weak due to the ample supply of moisture, and typically do not persist for too long, leading to short durations.

11. Why is the cybernetic approach to the hydrologic cycle better than the cause-effect approach? What fact of Nature's functioning supports the argument in favor of the cybernetic approach?
Because the cybernetic hydrologic cycle is more in tune with the way Nature works, with biofeedback as its controlling mechanism. Nature is not built on cause-effect, had this been the case, it would have long ago risen out-of-control along a J-curve and finished or changed. Cybernetic implies self-control, which is what Nature does well. Example: Floods follow droughts and droughts follow floods. If droughts did not follow floods, the ecosystem would become too humid and would change status, therefore, cease to exist as such.