4. MATCHING WATER NEEDS AND SUPPLY

4.1 Introduction
4.2 Command Area
4.3 Matching SIN_gross, and SWS

4.1 Introduction

Chapter 3 of the present manual detailed the methods of estimating the irrigation need of a scheme. Chapter 2 discussed sources of irrigation water, and that it is often limited. This chapter will explain why the irrigation need and the irrigation supply should always be compared, particularly when the important elements in the scheme are likely to change, or when water shortages occur, as is all too frequently the case. A number of methods are discussed which can help diminish the danger of water shortages. This part of the manual is intended as a guide to the practical problems of water shortage. Moreover, the information offered here should provide field technicians with a water management tool with which to anticipate and solve irrigation problems at the more complex level of the agricultural scheme.

4.2 Command Area

The command area, CA, of a water source is the extent of area which can be reliably irrigated from that source. Reliable irrigation means that the availability of water is always larger than or equal to the irrigation need of a scheme.

If the command area of a water source is larger than the actual area to be irrigated, there should be no problem of water shortage. When the supply of a water source in a certain month is known, e.g. 250 litres per second, and the gross irrigation need per hectare for the same month is estimated, for instance at 1.8 litres per second per hectare, then the command area can be calculated. With a water supply of 250 l/s and a need of 1.8l/s per hectare, one can irrigate 250/1.8, or 139 ha. The formula used to calculate the command area is scheme water supply (SWS) divided by gross irrigation need (IN_gross) or

When the supply of a water source is not constant over the months and seasons of the year, then the command area will also vary. Imagine, for example, a SWS from a creek with the monthly values indicated in the schedule below (these values will be used for all of the examples throughout this chapter). In Chapter 3, the methods for determining the IN_net were explained. As the amount of water supply varies over the years, the irrigation need also varies. Assume also the following values of IN_net for each month, which need to be calculated as was done in 3.4.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
SWS (l/s)	290	420	510	560	650	400	320	280	250	230	200	220
INnet (l/s)	0.5	0.5	0.7	1.0	1.1	1.2	1.1	1.0	0.7	0.6	0.5	0.5

Assuming a scheme irrigation efficiency of 50%, values of IN_gross and the command area can be calculated (see Table 2).

Table 2 EXAMPLE OF MONTHLY COMMAND AREAS

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
SWS (l/s)	290	420	510	560	650	400	320	280	250	230	200	220
INnet (l/s/ha)	1.0	1.0	1.4	2.0	2.2	2.4	2.2	2.0	1.4	1.2	1.0	1.0
CA (ha)	290	420	364	280	295	167	145	140	179	192	200	220

The month during which the smallest area can be irrigated, i.e. the one with smallest CA, is August, with 140 ha. The month with the largest CA is February with 420 ha.

Suppose that the farmers grow sugar cane on an area measuring 420 hectares. This crop has a growing period of 12 months. In this case, only during one month, February, would there be enough water available to irrigate that total area. In other words irrigation would be highly unreliable. If planting were confined to an area of not more than 140 ha, the sugar cane could be supplied with sufficient water throughout the year. Irrigation in this case is reliable. The area of 140 ha is in fact the smallest area according to the calculations in Table 2. This area is called the critical command area.

Now, suppose that the farmers grow a rice crop instead of sugar cane. The rice has a growing period of 4 months and is planted in April and harvested at the end of July. The IN_gross of rice is assumed as in the Table 3. The monthly command areas for this growing season are calculated in Table 3, the same as was done in Table 2.

Table 3 EXAMPLE OF MONTHLY COMMAND AREAS FOR A RICE CROP

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
SWS (l/s)	290	420	510	560	650	400	320	280	250	230	200	220
INgross (l/s/ha)	2.0	2.0	2.0	2.0	2.4	2.6	2.4	2.2	2.0
CA (ha)				280	271	154	133

The critical command area in this example measures 133 ha during the month of July, so that the area which can safely be planted in April should not be any greater than 133 ha, even though the command area for that month is much larger.

SAMPLE PROBLEM: Determine the critical command area for a crop of beans. ASSUMPTIONS: Growing period for beans is September, October, and November Net irrigation need, INnet, is 6, 5 and 4 mm per day, respectively Irrigation efficiency is 50 %
SWS			September: 250 l/s
SWS			October: 230l/s
SWS			November: 200 l/s
CALCULATIONS: Step 1: Convert the net irrigation need in mm/day to net irrigation need in l/s/ha. According to the conversion table in Annex II, 6, 5 and 4 mm/day can be converted into 0.69, 0.58 and 0.46 l/s/ha, respectively. Step 2: Determine the gross irrigation need INgross. INgross = INnet X 100 / e
			INgross (l/s)
Sept.			1.4
Oct.			1.2
Nov.			0.9
Step 3: Determine the command area per month. CA = SWS / Ingross
CA September	= 250/1.4	= 179 ha
CA October	= 230 / 1.2	= 192 ha
CA November	= 200 / 0.9	= 222 ha
Step 4: Determine the critical command area. The critical command area is the smallest monthly command area of the growing period. CA = 179 ha

4.3 Matching SIN_gross, and SWS

In the foregoing section, IN_gross was defined as the gross irrigation need per hectare expressed in litres per second per hectare. When we know the total irrigated area of a scheme, we can multiply the IN_gross with the area, and we find the gross scheme irrigation need, SIN_gross

SIN_gross (l/s) =IN_gross (l/s/ha) x Area (ha)

The SIN_gross is the continuous water flow that the irrigation scheme needs, the SWS is the water flow that can be delivered by the water source. In this section, five possible means or methods are discussed of matching the SIN_gross with the SWS, when the irrigation supply is less than the amount required.

Method 1: Enlarge the SWS if the SIN_gross is too large.

Enlarging the SWS is only possible when the water source is not fully exploited. This may be the result of a pumping system that does not have enough power to lift the required water flow, or an intake structure or supply canal that is too small or placed too high above the water level in the river or reservoir. These situations can be improved and the supply increased. Of course, installing a new pump or intake will entail expenditures and other efforts. Enlarging the SWS is impossible when the river flow itself is too small, except unless a very expensive reservoir is constructed.

If irrigation water is being pumped from a well, it may be the case that the capacity of the well is limited. In this case, the well has to be deepened, or a second well and pump installed.

Method 2: Diminish the required SIN_op by changing the operational criteria.

When irrigation water is supplied intermittently, the water source is not used full-time. For instance, river water flows away during the hours or days that the river is not tapped. The exploitation of the river is not optimal in such a case. Moreover, when irrigation applications are prolonged or irrigation intervals are shortened, the required SIN_op will diminish,

If, however, the pump is already used most of the time, or farmers do not want to change the irrigation schedule, which are related to the operational criteria, another solution must be found.

EXAMPLE: Suppose the pump has a capacity of 315 l/s. The SINgross of the scheme is 100 l/s. The pump is operated 8 hours per day, 5 days per week. According to Section 3.5, the SINop should equal the SINgross divided by the operational criteria, Top. SINop = SINgross / Top = 100 /((8/24) x (5/7)) = 420 l/s With this operational criteria, the pump can not meet the required discharge of 420 l/s. If, on the other hand, the pump is used 9 instead of 8 hours per day and 6 instead of 5 days per week, then the required discharge will be: SINop = 100 / ((9/24) x (6/7)) = 311 l/s. This is within the pump's capacity.

Method 3: Matching by advancing or postponing the growing season.

In the case of a variable river flow, the SWS might be too low at the beginning or at the end of the growing season. If a water shortage occurs at the beginning of the season, one may consider postponing the growing season by a month or so. If water problems are more likely to occur at the end of the season, it may help to plant the crop earlier.

EXAMPLE: Take an irrigation scheme of 150 ha and a scheme irrigation supply pattern as given in Table 3. The farmers in the scheme grow rice throughout the entire area. The INgross values are assumed as in Table 3 and the growing season lasts 4 months. Farmers start cultivation in April. Table 4 contains the monthly command areas for the rice crop. Table 4 EXAMPLE OF MATCHING BY ADVANCING THE GROWING SEASON
Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
SWS (Ifs)	290	420	510	560	650	400	320	280	250	230	200	220
INgross (l/s/ha)		2.0	2.0	2.0	2.4	2.6	2.4
CA (ha)		210	255	280	271	154	133
As can be seen in Table 4, the SWS for the months April, May, and June is more than enough to irrigate 150 ha, but in July the water is enough for only 133 ha. If, however, farmers had started the rice crop in March instead of April, the critical command area would have been the 154 ha of the month of June and no water shortage would have occurred. If farmers planted in February, the critical command area becomes 210 ha for the 4-month rice growing season.

Method 4: Stagger the growing season.

Method 3 consisted of either advancing or postponing the growing period for the entire irrigation scheme. By doing so, it is sometimes possible to avoid the risk of water shortages at the beginning or at the end of the irrigation season. With Method 4, the growing season is shifted, but only for a part of scheme. In planning this operation carefully, the SIN_gross can be fine-tuned to an even greater extent to the SWS.

EXAMPLE: Suppose that the SWS and INgross of a rice scheme are again like that in Table 3. The possible growing season for rice runs from February to August, and the growing period lasts 4 months. The scheme area is 250 ha. First, the monthly command areas are determined. Table 5a MATCHING BY STAGGERING THE GROWING SEASON WITH ADVANCED CROPPING
Month	Jan		Feb		Mar			Apr			May			Jun			Jul			Aug		Sep			Oct			Nov			Dec
SWS (l/s)	290		420		510			560			650			400			320			280		250			230			200			220
INgross (l/s/ha)			2.0		2.0			2.0			2.4			2.6			2.4			2.2
CA (ha)			210		255			280			271			154			133			127
In looking at the calculated command areas, one can see that there is no consecutive four-month period in which the critical command area is greater than 250 ha. For rice planted in February and harvested in May, there is enough water available, throughout the entire growing period, for 210 ha. In this case, all the water available in February is used. In March, April, and May some water is left over. And in June, July and August no water is consumed. If we subtract the planted area, 210 ha, from each of the monthly command areas, the differences will be the command areas that are left over unused. Table 5b MATCHING BY STAGGERING THE GROWING SEASON WITH ADVANCED CROPPING
Month	Jan			Feb			Mar			Apr			May			Jun			Jul		Aug			Sep			Oct			Nov			Dec
SWS (l/s)	290			420			510			560			650			400			320		280			250			230			200			220
INgross (l/s/ha)				2.0			2.0			2.0			2.4			2.6			2.4		2.2
CA (ha)				210			255			280			271			154			133		127
early rice (ha)				210			210			210			210
CA left (ha)				0			45			70			61			154			133		127
Now the smallest of these left-over command areas is the critical command area for the postponed rice growing season. In this case it amounts to 45 ha, which can then be planted later, in March, in addition to the 210 ha which are planted in February. If the late rice season were to be postponed until April or May, the critical command area would become 61 ha, as can be seen in Table 5c. Table 5c MATCHING BY STAGGERING THE GROWING SEASON WITH ADVANCED CROPPING
Month	Jan	Feb				Mar			Apr			May			Jun			Jul			Aug		Sep			Oct			Nov			Dec
SWS (l/s)	290	420				510			560			650			400			320			280		250			230			200			220
INgross (l/s/ha)		2.0				2.0			2.0			2.4			2.6			2.4			2.2
CA (ha)		210				255			280			271			154			133			127
early rice (ha)		210				210			210			210
CA left (ha)		0				45			70			61			154			133			127
late rice (ha)									61			61			61			61
CA left (ha)		0				45			9			0			93			72			127
The total possible area where rice can be planted safely is thus 210 + 61 = 271 ha. This exceeds the total scheme area of 250 ha.

In the example priority was given to planting as large an area as possible as soon as possible. This is the best solution if farmers on the scheme want to start the rice season as early as possible. However, if they were to prefer planting the rice as late in the season as possible, and harvest in July or August, we will need to determine the critical command area for the last 4 months. Areas where there is not enough water available during the last four months, should be planted in advance. Table 6 below shows the results of the calculation made.

Table 6 MATCHING BY STAGGERING THE GROWING SEASON WITH POSTPONED CROPPING

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
SWS (l/s)	290	420	510	560	650	400	320	280	250	230	200	220
INgross, (l/s//ha)		2.0	2.0	2.0	2.4	2.6	2.4	2.2
CA(ha)		210	255	280	271	154	133	127
latest rice (ha)					127	127	127	127
CA left (ha)		210	255	280	144	27	6	0
late rice (ha)				6	6	6	6
CA left (ha)		210	255	274	138	21	0
early rice (ha)			21	21	21	21
CA left (ha)		210	234	253	117	0
earliest rice (ha)		117	117	117	117
CA left (ha)		93	117	136	0

The total irrigable area is 127 + 6 + 21 + 117, which comes to a total of 271 ha, the same as in the first example. Although the end result is the same, the second example's growing season has to be staggered more. There are, in fact, many other ways of splitting up a scheme into sub-units with different growing seasons.

Method 5: Diminish the SIN_gross.

If all other methods of coping with water shortage fail, the only remaining solution is to diminish the gross scheme irrigation water need. The first step in decreasing the SIN_gross is to try to increase irrigation efficiency. More information on this subject will be provided in the following training manuals.

If irrigation efficiency is still not sufficiently improved to match need with supply, or SIN_gross and SWS, three alternatives remain:

- grow a crop with a lower irrigation need
- decrease the irrigated area
- accept water shortages and lower production.

In existing schemes, farmers will not easily accept growing other crops or diminishing the irrigated area. When the water shortage is less than 10-20% of the monthly requirement, production losses are not very heavy, and farmers will accept this solution first. When an extension of the scheme is considered, and farmers want to expand beyond the limits of the available water, one has sometimes to convince them to adjust their plans, by cutting a part of the new area, or by growing a crop with a lower irrigation need.