(a) What are the Hadley and Ferrell cells? (b) How do they affect precipitation across northern latitudes?

(a) The Hadley and Ferrell cells are two large-scale atmospheric circulation patterns.

(b) The Hadley cell rises at 0°, causing increased precipitation in tropical latitudes, moves north in the Northern hemisphere, descending at about 30°; thus, causing decreased precipitation at midlatitudes. The Ferrell cell is the opposite of the Hadley cell, rising at 60° (temperate latitudes), increasing precipitation at temperate latitudes, moving south in the Northern hemisphere, and descending at about 30°; thus, decreasing precipitation at midlatitudes.

(a) What is the main conceptual difference between the conventional water balance and that of L'vovich? (b) What is the main drawback of the conventional method?

(a) The main difference is the concept of wetting, which in L'vovich's approach is defined as the fraction of precipitation not converted to direct surface runoff.

(b) The main drawback of the conventional water balance method is the double counting of infiltration and baseflow, because infltration may follow either of two paths: (i) remain close to the surface, to eventually form part of evaporation or evapotranspiration, or (ii) move vertically down and reach the groundwater proper, eventually to join a neighboring stream and add to total runoff in the stream (surface and subsurface).

Describe four (4) important properties of water that make it unique.

The unique properties of water, that make it unique when compared to other molecules, are the following:

• Its capacity for ambient temperature regulation.

• Its floatability in solid state.

• Its surface tension and capillarity properties.

• Its marked solvent property (water will dissolve almost anything).

• Its active relation with proton chemistry (pH) and electron chemistry (pE, or redox potential).

(a) What is the main difference between the Penman and Penman-Monteith methods for the calculation of evaporation? (b) Did the Shuttleworth-Wallace method improve on the Penman-Monteith method? At what cost?

(a) In the Penman method, the mass-transfer evaporation rate is calculated based on an empirical formula. On the other hand, in the Penman-Monteith method, the mass-transfer evaporation rate is calculated based in physical principles, using the concepts of (1) external (aerodynamic) resistance, and (2) internal (stomatal, or surface resistance).

(b) Yes, the Shuttleworth-Wallace method did improve on the Penman-Monteith method by defining up to five (5) resistances to better characterize canopy and substrate, instead of the only two (2) established by Monteith. However, the additional complexity and data needs has limited the applicability of the Shuttleworth-Wallace method.

(a) How did Mockus arrive at his famous equation for soil loss? (b) What does the equation do?

(a) As Vic Mockus told Prof. Ponce in the now famous interview: "I arrived at the (runoff curve number) equation one evening, after dinner, seeing that it fitted the data really well, and after having tried many other alternative relations."

(b) Indeed, Mockus' runoff curve number equation is a nonlinear fit to the rainfall-runoff (P-Q) data. For a given soil, of certain curve number CN, as P increases, the amount of soil storage (P - Q) asymptotically reaches a maximum value S.

What sets Mockus' method apart from other soil infiltration formulas such as Green and Ampt? What is missing from the physically based infiltration models?

(a) While Mockus' method has a finite value of maximum soil storage (S), the Green and Amp infiltration formula has no such limit, featuring instead an infinite amount of soil storage. Therefore, while Mockus' method is suited for modeling saturation overland flow, the Green and Ampt formula is better suited for a bottomless infiltration model, such as Hortonian overland flow.

(b) Most soils have a practical limit on depth. This is missing from the physically based models such as Green and Ampt's.

(a) How does baseflow vary across the climatic spectrum, from superarid to superhumid? (b) Why?

(a) In superarid regions, the streams are typically ephemeral, with no baseflow. Conversely, in superhumid regions, the streams are perennial, with baseflow constituting a large portion of the total discharge.

(b) The reason why baseflow varies from near zero at the dry extreme of the climatic precipitation spectrum, to very substantial in superhumid regions, is the position of the groundwater table, relative to the stream bottom. In arid regions, the water table lies below the stream bottom, while in humid regions, the situation is reversed, and the stream bottom lies below the water table.

(a) What four (4) variables are included in the formula for theoretical time of concentration in overland flow? (b) Which are in the numerator? Which are in the denominator?

(a) The variables that are included in the formula for theoretical time of concentration in overland flow are the following: (1) plane length L, (3) roughness coefficient (Manning's n), (3) plane slope S, and (4) effective rainfall intensity i_e .

(b) Plane length L and n are in the numerator; thus, time of concentration increases with an increase in L and n. Slope S and i_e are in the denominator; thus, time of concentration decreases with an increase in S and i_e .

(a) What is surface albedo? (b) How does surface albedo affect the amount of precipitation?

(a) Surface albedo is the coefficient of reflectivity of a given surface toward short-wave radiation, i.e., the ratio of reflected to incoming radiation.

(b) Lower albedos absorb more solar radiation; higher albedos absorb less solar radiation. Thus, a surface with a low albedo, such as a rain forest (a = 0.07) will release more energy as heat than a surface with a high albedo, such as a desert (a = 0.45). The additional lifting of air masses produced by the heating (from below), due to the low albedo, will generally lead to additional (more) precipitation.

(a) What is El Niño? (b) To what do atmospheric scientists attribute the root cause of El Niño?

(a) El Niņo is a global climatic anomaly produced by a weakening of the trade winds along the tropical Pacific, which leads to warmer-than-normal sea-surface waters along the eastern tropical Pacific.

(b) The root cause of El Niņo is not known with certainty. Atmospheric scientists attribute it to large-scale teleconnections emanating from the warm-water regions of the mid-Pacific along the equator.

Why is drought persistence (drought duration lasting more than one year) likely to peak around the middle of the precipitation spectrum?

Drought persistence is likely to peak around the middle of the climatic spectrum (the limit between semiarid and subhumid climates, with 800 mm of mean annual precipitation), because in an arid climate, to the left of the spectrum, droughts are more frequent, therefore, necessarily of short duration, and consequently of limited intensity. Conversely, in a humid climate, to the right of the spectrum, with typically feature more than four (4) months of rainy season, droughts are less frequent and typically of short duration and, consequently, of limited intensity. Thus, drought intensity and, thus, persistence, is poised to be greater around the middle of the climatic spectrum.

Describe the concept of the 800-mm isohyet and its relation to the principle of sustainability.

The concept of the 800-mm isohyet arises from the observation that in arid regions, water is limited due to the scarcity of rainfall, while nutrients abound in the geologically relative new soils. Conversely, in humid regions, the situation reverses; nutrients are limited due to soil leaching through millennia, but there is ample water due an abundance of rain. Thus, the optimum situation for both water and nutrient availability appears to be in the middle of the climatic spectrum, i.e., at the isohyet corresponding to the mean global terrestrial precipitation, estimated at 800 mm (31 in). In this location, water availability should be ample enough to reduce the need for transport (hydraulic engineering). Moreover, the salts that usually plague the development of arid lands (with geologically new soils) for irrigation, would be reduced to manageable levels. The reduced need for both water conveyance and salt management enhances ecosystem sustainability.