[A] The Referencing System
[B] Constraints on Land Classification and Use (Precipitation, Elevation, Slope, Climate, Soil Type, etc.)
[C] Productivities of Various Categories of Land
[D] Nation- and Region Area data 

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Reference citations in this compilation look something like (df98H3). You should interpret this notation as follows. There is no separate reference list for this document. Instead, the reference lists used in the soils degradation review (se), the forest lands degradation review (df), the grazing lands degradation review (og), the irrigated lands degradation review (ir) and the fisheries degradation review (fi) are used, since all the data compiled in this document come from one of these five reviews. Thus the notation (df98H3) means that you should first go to the reference list for the forestland degradation review (df). Then look for the reference citation labeled "98H3" which means the third reference published in 1998 by a lead author whose last name begins with "H". Note that the reference list for the soils degradation review (se) is divided into two separate files - one for references published through 1986 and another for references published after 1986. Throughout this document you will find links to the six files that contain these separate reference lists. Each of these reference lists is divided by bookmarks into small ranges of years to facilitate your search for a given reference. The links to references throughout this document contain these bookmarks.

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• Precipitation above 35 cm./ year makes continuous dryland farming possible (se70P3). (la)
• 20"/ year (51 cm./ year) of precipitation is considered to be the minimum for dryland farming in Colorado ((se81S1), p. 77). (la)
• Croplands (with less biomass per unit-area of land than forests) become marginal at about 20" (50 cm./ year) of precipitation yearly. (la)
• In dry climates, fallowing may be required to maintain soil moisture on croplands.
• Wind erosion is a major problem when attempting to farm in dry climates.

• Forests generally occur only where precipitation exceeds 60 cm/ year (24"/ year) (Ref. 64 of (df91M1)). (la) 

• 20-25% slope (9-11^o) is generally considered the maximum that can be cropped in the US ((se74C1), p. 70). (la)
• Chinese government policy prohibits cultivation on slopes over 28% (se82B2). (la) 

Climatic Constraints on Arable Land:
Of the 130 million km² of ice-free land in the world, 26 million are not arable because the temperature is below freezing over 9 months of the year. In an added 19 million, there are fewer than three months of the year when available moisture from rain, snow, or water stored in the soil exceeds potential transpiration from plants and soil, and there are no practical sources of irrigation water. Climate alone, therefore limits potentially arable land to (130 - 26 - 19) = 85 million km² (se76R1).
Comments: Also from this 85 million km² subtract urban lands (See se Chapter 6) and excessively steep lands. Other estimates of reasonably biologically productive land give about 90 million km² (See elsewhere in this document.) (la)

Elevation Constraints on Forestlands:
Land above 3000 m. elevation [about 10,000 ft.] is about 5% of the world's terrestrial surface (se97G2) (la). Comments: 148.9 * 0.05 = 7.45 million km² above 3000 m.

• Above tree-lines, land is almost certainly rocks, tundra, low shrub, etc. with little if any potential for serving as cropland, grazing land or forest land.
• Tree lines in the western US decrease from 3500 m. (11,500 ft.) in the southwest to 2000 m. (6600 ft.) in northern Montana.
• In Alaska, tree lines occur at less than 1000 m. (less than 3280 ft.).
• In the northeastern US, tree lines occur at 1500 m. (4900 ft.) (Ref. 39 of Ref. (df78B2)). (la)
• The usual definition of timberline is that it's where there are 2 months or less of average temperatures over 50 degrees F (George Wuerthner, posting to RangeNet@egroups.com, 9/17/00). (la) 

Main Soil Constraints in the Amazon Basin ((se82B1), Ref. 11) (la) (Areas are in millions of km² )

Also see Chapter 2, Section (2-A) [5] for data on biomass production rates and biomass inventories.

Dregne and Chou (Ref. 18 of (se97C1)) estimate the value of production of irrigated cropland at $62,500/ km²/ year ($9,500/ km²/ year for rain-fed cropland and $1,750/km²/ year for rangelands). (la) Comments: Rangelands trend to be semi-arid or arid, explaining their low productivity. Pasturelands tend to be found in semi-humid or humid climates on land that is too steep to use as cropland, or that has other constraints. Pastureland is generally able to support about a factor of 10 more cattle per unit area of land than rangeland. However it is probably less productive than rain-fed cropland since one rarely sees level land being used as pastureland. 

Riparian Productivity: Riparian zones typically represent 2-5% of a grazing area. But they may supply up to 80% of forage used by cattle (og94O1).

Comments: A table of major deserts of the world (Chapter 6 in this document) gives about 15 million km² of desert, but hyper-arid regions cover only about 5.9 million km² (se85D2).
Comments: The total above should be about 132,000 for the world's ice-free land, and (total+ polar) should be about 148,000 for all land, including ice-free. Perhaps mountains above tree line have been neglected. Also, deserts seem to be under-estimated. See Chapter 6 ("Arid Lands and Other Biologically Non-productive Lands") in this document.
Comments: Other estimates of ice-covered land give 148,000 - 132,000 = 16,000 km²
Comments: "Steppe" is defined as arid land with vegetation adapted to low moisture availability (xerophilous) usually in treeless regions of extreme temperature range and loess (wind-blown) soil.

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* Line 1 for each region: total of individual nation data.
*# Line 2 for each region: regional total from Table 17.1 of (se90W1).
*## Line 3 of final totals: final totals from Table 17.1 of (se90W1).