Worksheet Descriptions

The worksheet contains 10 tables that require data, as well as various individual cells for specific information. Information requests are described below.

Prior to Table 1

1. Total project area: This is the gross project area (ha), including fields that are supported by a project water delivery infrastructure ("command") and fields that are not supported by the infrastructure.

2. Total field area in the command area: This is the number of hectares that are supported by a project water delivery infrastructure. There may be some zones of this command area that never receive water because of infrastructure damage, due to shortage of water, etc.

3. Estimated conveyance efficiency for external water:

   

   Where, in this case, the "point of delivery" is where farmers take control of the water - that is, where the Water User Association and Project Authorities hand the water over. Sometimes a turnout (offtake) represents the final point of delivery by an irrigation authority, yet that turnout supplies 100 fields.

   Conveyance losses include seepage, spillage, water lost in filling and emptying canals, evaporation from canals, evapotranspiration from weeds along the canals, etc.

   The conveyance efficiency includes losses that occur between the point of original diversion and the entrance to the command area, which in some cases may be many kilometers apart.

4. Estimated conveyance efficiency for internal project recirculation.
   This is the conveyance efficiency for water that originates within the project, by project authorities. That is, it includes water that the agency pumps from wells or drain ditches or other internal sources. It does not include any water that is imported into the project boundaries.
   

5. Estimated seepage rate for paddy rice.
   There will only be an answer here if paddy rice is grown in a project. This is the percent of water applied to fields that goes below the root zone of the rice. Seepage rates are often expressed in mm/day, in which case they must be converted to a percentage of the field-applied irrigation water.
   Many studies combine "seepage" together with "evapotranspiration" for rice, to come up with a combined "consumptive use". That convention is not used in RAP, because such a combination makes it very difficult to separate ET (which cannot be recirculated or reduced) from seepage water (which can be recirculated via wells or drains). Furthermore, such a convention ignores the fact that deep percolation is unavoidable on all crops, not just on paddy rice. Therefore, the convention would apply to all crops, not just paddy rice.

6. Estimated surface losses from paddy rice to drains.
   There will only be an answer here if paddy rice is grown in a project. This is the percentage of irrigation water applied to fields, or groups of fields that leaves the fields and enters surface drains. This does not include water that flows from one paddy into another paddy…unless it ultimately flows into a surface drain.
   

7. Estimated field irrigation efficiency for other crops.
   This is an estimate for non-rice crops. The elements of inefficiency for paddy rice (deep percolation and surface runoff losses) have already been dealt with.
   The term "irrigation efficiency" has a rigorous definition (Burt et al., 1997). But the nature of a RAP is such that the values required for the rigorous application of the definition will not be available. Therefore, for the purposes of this RAP,
   
   

   

   where

   • the only water considered in the numerator and denominator is "irrigation" water. Water from precipitation is not included, since this indicator is a measure of how efficiently irrigation water is used.

   • "Special practices" include water for leaching of salts, land preparation, and climate control. However, for each of these categories, there is an upper limit on the amount that is accepted as beneficial use (and that can be included in the numerator). The RAP computations include an estimate of actual leaching requirement needs. The water assigned for land preparation for rice should not include excess deep percolation (caused by holding water too long on a field) or water that flows off the surface of a field.

   • For crops such as rice, which are often farmed as a unit that includes several fields that pass water from one field to another, "field" efficiency can be based on the larger management unit of several smaller field parcels.

   In general, this value is a rough estimate. The spreadsheet computes a correct value of "field irrigation efficiency" in the worksheet "4. External Indicators" (Indicator No. 31), which should be compared against this assumed value.

   This value is only used for one purpose in the spreadsheet: To estimate the recharge to the groundwater from field deep percolation. If, upon completion of the RAP, this estimate is different from the computed estimate, the RAP user should adjust this assumed value (and/or the rice deep percolation and surface runoff values) until Indicator 2 approximately equals Indicator 31.

8. Flow rate capacity of the main canal(s) at diversion point(s).
   This value should reflect the sum of the actual (as opposed to "design") maximum flow rate capacities from each diversion point. Sometimes the actual capacities are higher than the original design capacities, and in other cases they have been reduced due to siltation or other factors.
   

9. Actual peak flow rate into the main canal(s) at the diversion point(s).
   The purpose of this question is to define the maximum flow rate of irrigation water that enters the project boundaries. It should not include any internal pumping or recirculation of water.
   

10. Average ECe of the Irrigation Water.
    If possible, this "average" should be the annual weighted average, based on the salt load (ppmflow ratetime). It should be computed as a combination of the well water and surface water.
    

Table 1 - Field Coefficients and Crop Threshold ECe.

1. Water Year Month. The table provides 12 cells at the top of the Field Coefficient section into which the names of all 12 months are to be placed. Although the table could have had a default month of "January" in the first cell, many projects have "water years" that begin at other months - such as April in Southeast Asia or October or November in Mexico. Place the appropriate month in the highlighted empty cell to begin the water year accounting.

2. Irrigated Crop Name
   This column allows the user to input the names of the irrigated crops in the command area. A total of 17 crops are allowed, although the first 3 are already assigned to "Paddy Rice", leaving 14 other names blank for the user. Although a command area may have more than 17 crops, in general many of these crops have small areas of cultivation and for practical purposes can be lumped together as a single crop category.
   
   

If a crop is double cropped, then that crop name should be entered twice. The table already has default names for 3 paddy rice crops, because so many projects have 3 or more rice crops per year. You cannot over-ride the paddy rice crops; you cannot substitute other names for these 3 entries because certain computations assume rice in these cells.

Crop names only need to be entered once - into Table 1. They are automatically carried into all other tables that require crop names. This ensures consistency between tables.

3. Salinity.

   1. Average Irrigation Water Salinity (ECw), dS/m. The average salinity of the irrigation water that comes into the project. The units of dS/m are equivalent to mmho/cm.

   2. Threshold ECe, dS/m. This is the salinity of a saturated soil paste extract at which a crop yield will begin to decline. Example values are found in Table A.

Table A. Salt tolerance of various crops to soil salinity, after germination. (After Maas and Hoffman, 1977).


Crop	Threshold ECe (ECe at initial yield decline) dS/m	Crop	Threshold ECe (ECe at initial yield decline) dS/m
Alfalfa	2.0	Onion	1.2
Almond	1.5	Orange	1.7
Apricot	1.6	Orchard grass	1.5
Avocado	1.3	Peach	1.7
Barley (grain)	8.0	Peanut	3.2
Bean	1.0	Pepper	1.5
Beet, garden	4.0	Plum	1.5
Bermuda grass	6.9	Potato	1.7
Broad bean	1.6	Rice, paddy	3.0
Broccoli	2.8	Ryegrass, perennial	5.6
Cabbage	1.8	Sesbania	2.3
Carrot	1.0	Soybean	5.0
C Clover	1.5	Spinach	2.0
Corn (forage and grain)	1.8	Strawberry	1.0
Corn, sweet	1.7	Sudan grass	2.8
Cowpea	1.3	Sugar beet	7.0
Cucumber	2.5	Sugarcane	1.7
Date	4.0	Sweet potato	1.5
Fescue, tall	3.9	Tomato	2.5
Flax	1.7	Wheat	6.0
Grape	1.5	Wheat grass, crested	3.5
Grapefruit	1.8	Wheat grass, tall	7.5
Lettuce	1.3

The leaching requirement (LR) for each crop is computed within the spreadsheet as:



where ECiw = EC of the irrigation water, dS/m
         ECe = Threshold saturated paste extract of the crop, dS/m

For example, if ECiw = 1.0 dS/m
                    Crop = grain corn
From Table A, ECe = 1.8 dS/m

LR =


The extra water required for each crop, to remove salinity that arrives with the irrigation water, is then computed as:

Extra water for salinity control =

For example, if for a specific crop,
                  ET of irrigation water = 100,000 MCM
                  LR = .125
                  The volume of water needed for salinity control = 14,286 MCM

However, deep percolation of rainwater will accomplish the same task (it washes accumulated salts out of the root zone). Therefore, this RAP approximates the irrigation water requirement as:

Volume of irrigation water needed for salinity control
           = Volume of water needed for salinity control
              - Rainfall deep percolation

4. Field coefficients.
   Most irrigation specialists are familiar with the term "crop coefficient". Crop coefficients have been widely used in estimates of crop evapotranspiration (ET) since the mid-1970's. The general formula used is:

ETcrop = Kc ETo

where Kc = the crop coefficient
       ETo = grass reference ET

Guidelines for estimating ET and ETo are found in FAO Irrigation and Drainage Paper 56 - "Crop Evapotranspiration - Guidelines for computing crop water requirements" (Allen et al, 1998).

"Reference" values other than ETo are sometimes used, but they are quickly being replaced with weather stations that provide the hourly data needed to compute ETo. This spreadsheet uses ETo as defined in FAO 56 because

1. ETo is today's standard "reference"

2. The majority of excellent ET research on a variety of crops uses ETo as the reference crop.

3. ETo estimates tend to be more accurate than other reference methods such as evaporation pans.

If the only local data is from evaporation pans, it is recommended that users consult with FAO 56 to determine the proper conversion from monthly Epan to monthly ETo values. The table below comes from page 81 of FAO 56, where

ETo = Kp Epan

Table B. Pan coefficients (Kp) for Class A pan for different pan siting and environment and different levels of mean relative humidity (RH) and wind speed (FAO 56)

Class A Pan Description -›	Case A: Pan placed in short green cropped area				Case B: Pan placed in dry fallow area
RH mean (%) -›		low (<40)	medium (40 - 70)	high (>70)		low (<40)	medium (40 - 70)	high (>70)
Wind speed (m s-1)	Windward side distance of green crop (m)				Windward side distance of dry fallow (m)
Light (<2)	1	.55	.65	.75	1	.7	.8	.85
	10	.65	.75	.85	10	.6	.7	.8
	100	.7	.8	.85	100	.55	.65	.75
	1000	.75	.85	.85	1000	.5	.6	.7
Moderate (2-5)	1	.5	.6	.65	1	.65	.75	.8
	10	.6	.7	.75	10	.55	.65	.7
	100	.65	.75	.8	100	.5	.6	.65
	1000	.7	.8	.8	1000	.45	.55	.6
Strong (5-8)	1	.45	.5	.6	1	.6	.65	.7
	10	.55	.6	.65	10	.5	.55	.65
	100	.6	.65	.7	100	.45	.5	.6
	1000	.65	.7	.75	1000	.4	.45	.55
V. Strong (>8)	1	.4	.45	.5	1	.5	.6	.65
	10	.45	.55	.6	10	.45	.5	.55
	100	.5	.6	.65	100	.4	.45	.5
	1000	.55	.6	.65	1000	.34	.4	.45

This spreadsheet uses the term "field coefficient" because quite often a "crop coefficient" is only used during the crop-growing season, and quite often the common usage of "crop coefficients" ignores the impacts of soil moisture contents.

In reality, the "field coefficient, Kc" is the same as the "crop coefficient, Kc" if the crop coefficient is properly adjusted (using FAO 56 guidelines) to include factors such as

* stress (reduced transpiration) due to a dry root zone
* soil surface evaporation due to rainfall or irrigation.

The proper selection of Field Coefficients depends upon a good understanding of Table 8 in the INPUT spreadsheets (Precipitation, effective precipitation, and deep percolation of precipitation). The computation procedure that the spreadsheet uses includes the following:

1. Effective precipitation and irrigation water are assumed to be the only external sources of water for field ET.

2. The field ET is computed on a monthly basis as:

ET = Kc ETo

Effective precipitation includes all precipitation that is lost through either evaporation (from the soil or plant) or transpiration, as computed by the formula above. Therefore, if one wants to account for soil evaporation for those months when the crop is not in the ground, one must do 2 things simultaneously:

1. The effective precipitation must be computed to account for that evaporation, and

2. A field coefficient (Kc) of greater than 0.0 must be applied to those months.

The following procedure is recommended for RAP:

1. For crops with no irrigation water used for pre-plant irrigation. If for a month the crop has not yet been planted, or a crop is not in the field, assume that for that month,

   • crop coefficient = 0.0

   • effective rainfall that is reported for that month will only include water that is stored in the root zone for ET after the seeds are planted.

2. For crops that use irrigation water for pre-plant irrigation (e.g., rice field preparation, cotton pre-irrigation). Follow the procedure of (a) above until the irrigation water is first applied. Then do the following for each month until the crop is planted or transplanted:

   • crop coefficient > 0 to account for soil evaporation of both irrigation water and effective rainfall.

   • effective rainfall that is reported for that month will include water that is stored for ET after planting, plus the rainfall contribution to the soil evaporation prior to planting.

As an example, assume a case in which

• A pre-plant irrigation is applied to a field on the first day of the month.

• The crop will not be planted for another month.

• The soil remains bare and free from weeds for this month.

• The soil remains "dark" for 3 days after standing water disappears from the soil surface.

Table C indicates how to compute an average monthly Kc that properly takes into account the soil evaporation. Rules to follow include:

• The minimum value of Kc is typically 0.15

• If a soil surface is dark in appearance from moisture, even if there is no standing crop, a crop coefficient of 1.05 is appropriate.

• Most unstressed field crops (cotton, rice, corn) have a crop coefficient of approximately 1.1 once they have achieved 100% canopy cover.
  

Table C. Example computation of an average monthly Kc value for a month following a pre-plant irrigation, but prior to planting.

Day	Kc	Explanation
1	1.05	Irrigation - wet soil surface
2	1.05	2nd day of irrigation - wet soil surface
3	1.05	1st day after irrigation. No standing water. Soil surface still dark
4	1.05	2nd day after irrigation. Soil surface still dark
5	1.05	3rd day after irrigation. Soil surface still dark
6	0.7	4th day after irrigation.
7	0.5	5th day after irrigation.
8	0.3	6th day after irrigation.
9	0.15	7th day after irrigation.
10	0.15	8th day after irrigation.
11	1.05	Rain - wet soil surface
12	1.05	2nd day of rain - wet soil surface
13	1.05	1st day after rain. Soil surface still dark
14	1.05	2nd day after rain. Soil surface still dark
15	1.05	3rd day after rain. Soil surface still dark
16	0.7	4th day after rain.
17	0.5	5th day after rain.
18	0.3	6th day after rain.
19	0.15	7th day after rain.
20	0.15	8th day after rain.
21	1.05	Rain - wet soil surface
22	1.05	2nd day of rain - wet soil surface
23	1.05	1st day after rain. Soil surface still dark
24	1.05	2nd day after rain. Soil surface still dark
25	1.05	3rd day after rain. Soil surface still dark
26	0.7	4th day after rain.
27	0.5	5th day after rain.
28	0.3	6th day after rain.
29	0.15	7th day after rain.
30	0.15	8th day after rain.
Avg. Kc =	0.71	for this month of 30 days

Table 2 - Monthly ETo values

ETo (mm) values by month should be entered. See the earlier discussion regarding crop coefficients. Ideally, ETo should be computed on an hourly basis using the Penman-Monteith method, following the procedure by Allen, et al (1998).

Table 3 - Surface Water Entering the Command Area Boundaries (MCM).

All values for this table should be in units of million cubic meters (MCM), and should only include water that can be used for irrigation. In other words, flows from a river flowing through a command area that has no diversion structures or pumps would not be included. The table allows for 3 general categories of surface inflows:

1. Irrigation Water Entering from outside the command area. The MCM should be the total MCM at the original diversion point(s). Therefore, technically speaking it is not the MCM entering the command area. This category of "irrigation water" is the "officially diverted" irrigation water supply.

2. Other inflows from external source #2. This source can be defined by the RAP user, and can be a consolidation of several physical sources - but all placed in one category. However, these inflows must be accessed by users within the command area as an irrigation supply - either through diversion or through pumping from rivers.

3. Other inflows from external source #3. This has the same qualification as #2.

The key concepts for Table 3 are these:

1. Table 3 only includes surface volumes that enter from outside the command area boundaries.

2. The surface volumes are only included if they are volumes of water used for irrigation. For purposes of the RAP, External Sources #2 and #3 are considered irrigation water if they consist of water that individual farmers or groups of farmers divert or pump. Many projects have such supplemental supplies that do not enter the command area through designed and maintained canals, yet these supplies are important parts of the overall irrigation supply in the command area.
   The important value here is the volume of water that enters the command area, NOT the volume of water that is pumped from drains….since that may also include recirculation of spills and field runoff.

Table 4 - Internal Surface Water Sources (MCM)

Table 4 values do not represent original supplies of water, since the surface sources were already accounted for in Table 3. Rather, this is the volume of water that is recirculated or pumped from surface sources within the project. This may be water that originated from the irrigation canal and was spilled, deep percolated, or ran off from fields. The origin of the water is not the important thing in Table 4. Rather, the important feature for Table 4 is which entity diverts or pumps this non-canal water.

Table 5 - Hectares of Each Crop in the Command Area, by Month

Table 5 provides information on how much area is used for each crop during each month.

The Kc values for each crop are found in the row immediately above the row into which you must input the hectares of that crop. If a Kc value greater than 0.0 exists for a month for that crop, you must input the number of hectares associated with that crop, for that month.

Table 6 - Groundwater Data

These questions only need to be answered if groundwater is used by farmers or by the project authorities.

Groundwater accounting in irrigation projects frequently ignores external sources of groundwater, and the fact that much of the groundwater may simply be recirculated surface water. This RAP eliminates the double counting of recirculated water, which is what happens if groundwater is treated as an independent supply.

Table 6 recognizes that an aquifer may extend well beyond the confines of the command area.

The questions are divided into 2 categories - pumping from the aquifer within the command area, and pumping from the aquifer but outside the command area. Both areas must be considered if the aquifer is to be examined properly. The External Indicators and Benchmarking indicators do not utilize the external pumping information. However, frequently the pumping from outside the command area is completely dependent upon seepage and deep percolation from within the command area. In such a case, a "water conservation" program within the command area to minimize seepage may actually eliminate the water source for groundwater pumpers outside the command area. Of course, there may be considerations such as contamination of the groundwater as it passes through old marine sediments - increasing the salinity of groundwater as compared to surface water.

The "net" groundwater pumping within the command area can only be greater than or equal to zero, the way the spreadsheet is designed. The computations is this:

- Estimates of deep percolation from fields are made.
- Estimates of seepage from canals is made

These two, when combined, represent the recharge of the aquifer from external irrigation water.

Estimates are then made of the groundwater pumping that occurs within the command area - either by project authorities or by individual farmers. This groundwater pumping volume is then discounted for estimated losses. The result is an estimate of the groundwater that actually contributes to evapotranspiration.

The volume of groundwater that is used for ET is compared against the recharge from surface water supplies. If the recharge is greater than the ET of groundwater, then the "net" groundwater pumping = 0.0. If the ET of groundwater is greater than the recharge, the difference is the "net" groundwater pumping. In most projects, the "net" groundwater pumping will equal zero because typically the aquifer is recharged with the imported surface irrigation water.

Although groundwater pumping is an important aspect of recirculation of irrigation water, it is not a "new" supply of water any more than recirculation of surface water would be. However, recirculation of any type will increase the irrigation efficiency of the project - but it will not have any impact on the irrigation efficiency of the field units, unless the recirculation occurs on the fields themselves.

Table 7 - Precipitation, Effective Precipitation, and Deep Percolation of Precipitation.

The monthly gross precipitation (mm) is required at the top of the table. These values are generally easily obtained.

The other values are probably somewhat of a mystery to most users, although the concepts of effective precipitation and deep percolation are common concepts. The problem the user will have is in identifying proper values. Unfortunately, simple assumptions about deep percolation and the percentage of rainfall that is effective do not work for spreadsheets such as this, that are designed to be applied over a wide range of geography, each having vast differences in climates and crops.

Effective precipitation is defined as precipitation that is destined for ET (evaporation or transpiration) either this month or in the future.

Effective precipitation and deep percolation can be input in this table for any or all months, regardless of whether a crop is in the field that month. The deep percolation of rainfall is used for only one computation purpose: as a computed reduction of the amount of irrigation leaching water that is necessary to wash salts from the root zone.

IIn general, values for "effective precipitation" and "deep percolation" are not available as monthly values, and they are almost never available for individual crops. Nevertheless, it is important to make an estimate of these values.

As an aid to the spreadsheet user, the calculated ETfield (mm) values are carried forward from previous tables (these tables are found on the far right hand side of the pages of this Worksheet, and include computations using ETo and Kc values). Once the spreadsheet user inputs an estimate of the percentage of effective precipitation, a corresponding depth of effective precipitation appears in the next row.

In general, if there is a light rainfall during a month yet the ETfield is high, there will be very little deep percolation of rainfall. Conversely, if there is a large amount of rainfall and very little ETfield, then one can expect more deep percolation. Deep percolation will depend upon the soil type, also - sandy soils have more deep percolation than do clay soils. The deep percolation cannot exceed the quantity: (Precipitation - Effective Precipitation)

Table 8 - Special Agronomic Requirements (mm)

Only a few crops will have values in this table. The most notable crop is paddy rice.

As an example for a rice crop, assume the following:

Table 9 - Crop Yields and Values

Three types of input are needed:

1. The local exchange rate ($US/local currency)

2. Typical average yields of each crop, in metric tons per hectare.

3. The farm gate selling price of each crop, in (Local currency/metric ton).

Worksheet 4. External Indicators

This worksheet is a temporary holding place for some values and computations.

For the user, the primary usage of this worksheet is to enter confidence interval values.