Types of Land

We can divide land on the basis of the amount of water it receive. The underlying assumption is that upto a certain extent fertility of land grows with amount of water available to it. Desert having little water has little fertility whereas deltas of nile, euphrates, ganges and indus have high fertility due to water availability.

There are infact three sources of water, primary which is rain, secondary which is canal, teritiary which is ground water. I would not consider the teritiary source in this article for land classification because it is energy intensive.

On the basis of quantity of rain it gets a land can be divided into 7 categories:

>80 inches
as good as desert for food production

80 inches
not so fertile due to excess water, can only grow a few crops like rice, grass, poor quality

40 inches
average land

20 inches
the most fertile land

10 inches
average land

5 inches
poor quality land

<5 inches
desert

On the basis of canal water available the land can be divided into 3 categories:

0 acre-ft/acre

1 acre-ft/acre

2 acre-ft/acre

What is "inches of rain" and "acre-ft/acre" anyways?

Lets consider the middle case of 10 inches of rain falling on one acre. Since there are 2.54 cm in each inch therefore it means 25.4 cm tall water column standing on one acre. Lets round that off to 25 cm for easy calculation and understanding. One acre is 4840 sq yards which is equal to 4047 sq m. Lets round that off too to 4000 sq meters. So, 10 inches of rain roughly means 25 cm water standing on 4000 sq m. The volume is 1000 cubic meter water. I call this 1k land because it gets 1000 cubic meter water in a year. In my calculations 1k land can be any land that gets 1000 cubic meter water irrespective of the source of water, it not always have to be rain water to call it "1k land".

One acre-ft/acre canal water means 1 ft tall water column standing on 1 acre of land. Acre-ft is the general unit of measurement of canal water. 1 ft means 12 inches and one inch means 2.54 cm, so 1 ft means 30.48 cm. Round that to 30 cm. Multiply that with 4000 sq m as before and we get 1200 cubic meter water per acre.

Before ending this discussion lets find the cost of rounding. In case of rain water, if we multiply the actual 4047 sq m with the actual 25.4 cm we gets 1028 cubic meter water instead of the round off value of 1000 cubic meter. In case of canal water, if we multiply the actual 4047 sq m with the actual 30.48 cm we gets 1234 cubic meter water instead of 1200 cubic meter.

Effective Water

In addition of knowing how much water is available per acre one must also know how much water getting to land is actually available for crops. Ofcourse not all of it is, some rain fall in off season when its of no use to crops, water flowing in canals and before that in rivers get evaporated on their long (hundreds of miles) journey to farm etc. Lets calculate the effective water in both cases of rain water and canal water one by one.

In my part of world, that is, in south asia, 80% of all rain water falls in two months of sawan and badho, that is from 16th july to 15th september. This also used to be the time when the crops need water in the traditional one-crop-per-year farming. So, 80% of all rains gets to land when the crops need them. I assume for simplicity the same for the rest of the world. It means that the average case of 10 inches rain water per year which is roughly translated to 1000 cubic meters water per acre per year is 800 cubic meters effective water.

In case of canal water, in Punjab due to relatively cooler weather and more fertile land the evaporation rate of canal water is 25%, in Sindh due to relatively hotter weather and generally less fertile land and also due to longer distance the canal water has to travel to farm the evaporation rate of canal water is 33%. I, for simplicity, takes the general rate of evaporation 33% so that 1 acre-ft/acre water which is roughly translated into 1200 cubic meters water above means 800 cubic meters effective water to equate with 10 inches rain. Note that there is no off-season canal water distribution because canal water is essentially the rain water stored in dams and barrages distributed only when needed, so all of it gets to land when needed there is no discount of off-season water, the only discount is evaporation.

Lets go back to the discussion of categories of land. Excluding desert and desert-like lands where rain water is less than 5 inches or more than 80 inches, we get five categories of land on basis of rain water. We also have 3 categories of land on basis of canal water. So, taking union, we have 15 categories of land as follows:

 5 inches rain                    =  400 cu m effective 
 5 inches rain + 1 acre-ft canal  = 1200 ditto 
 5 inches rain + 2 acre-ft canal  = 2000 ditto

10 inches rain                    =  800 ditto
10 inches rain + 1 acre-ft canal  = 1600 ditto
10 inches rain + 2 acre-ft canal  = 2400 ditto

20 inches rain                    = 1600 ditto
20 inches rain + 1 acre-ft canal  = 2400 ditto
20 inches rain + 2 acre-ft canal  = 3200 ditto

40 inches rain                    = 3200 ditto
40 inches rain + 1 acre-ft canal  = 4000 ditto
40 inches rain + 2 acre-ft canal  = 4800 ditto

80 inches rain                    = 6400 ditto
80 inches rain + 1 acre-ft canal  = 7200 ditto
80 inches rain + 2 acre-ft canal  = 8000 ditto

So, in short, the amount of effective water available vary between 400 cu m to 8000 cu m, a factor of 20. What we need is not the average case but the typical case. The average case is useful when the good, fair and worst are all in the same proportion which is not a real life situation.

So what is a typical case? A typical case is the mode as in statistics, that is the most common situation. It not need be the mathematical average. Its the case which you are most likely to encounter in a given situation.

How to find the typical case in a exponential situation? Well, first what is the exponential situation? Exponential situation is the situation where the numbers vary in multiples, like a sequence of 1,2,4,8,16. Here the typical case is 4 which is the middle value. It is the situation which you are most likely to encounter.

In the discussion of category of land, the best approach is to combine the case of 10inches of rain with 1 acre-ft/acre canal water. Result is 1600 cu m effective water per acre per year. For a deeper discussion, in order to get a sustainable agriculture it is unwise to use the canal water on regular basis. The agriculture should be primarily based on only the rain water, keeping the canal water as a safety valve in case of emergencies. Emergencies in this case includes droughts, pest attacks etc. If we embed the safety valve, the reserve troops in the normal operations where would we fall in case of failures? It sure is inefficient but it is indeed very resilient. All the world agriculture before the 1950s was infact based on only rain water, canal water was used only in emergencies, in years when rainfall is less than normal. Other disadvantages of canal water would be discussed in a separate article.

In summary, in absence of canal water, the average effective water available is 800 cu m, the lower value is 400 and the higher value is 1600, so we get the simple pattern of 3 values.

Worst:    5 inches OR 0.5 acre-ft =  400 cu m eff

Typical: 10 inches OR 1.0 acre-ft =  800 ditto

Best:    20 inches OR 2.0 acre-ft = 1600 ditto
                        OR
         10 inches + 1.0 acre-ft  = 1600 ditto

Average: 40 inches OR 4.0 acre-ft = 3200 ditto
                        OR
         20 inches + 2.0 acre-ft  = 3200 ditto

Worst:   80 inches OR 8.0 acre-ft = 6400 ditto
         40 inches +  4.0 acre-ft = 6400 cu m ditto

Note that 3200 cu m case is taken as average because after a certain level excess water actually reduce fertility instead of increasing it.

Only a few of the possible combinations are given above for 3200 cu m and 6400 cu m.

The point is hidden in the amount of effective water available, not on the source of water.

For simplicity, some new terms are introduced, 2k means 2000 cu m nominal and 1600 cu m effective, 3k means 3000 cu m nominal and 2400 cu m effective and so on. In that sense, land types varies between 1k and 10k, 2k being most fertile, both 1k and 4k being average, both 8k and 0.5k being poor. More than 8k are extremely unlikely to be of any value to agriculture.

Another important thing to consider is the relation between intensity of rain water and availability of canal water. At fertile land, for example in punjab, both the availability of canal water and intensity of rain is higher than that in sindh. An increased amount of canal water applied to land increase the humidity of air which inturn results in more rain fall. Considering the feedback loops would increase complexity so are not discussed in detail in this article. Only take-home message from this is that it is more likely to get 2 canal water in a land that already has 20 inches rain than in land that has 5 inches rain because that would be a desert already having little or no access to any kind of river or canal.