Dependable Flow and Irrigation Water Requirement

Fundamental concept

Water Requirement and Water Availability Must Be Analysed Together

Irrigation planning contains two different but connected calculations. The first determines how much water the crop and irrigation system require. The second determines how much river water can be supplied reliably during the critical irrigation period.

Demand Side

Crop evapotranspiration, effective rainfall and irrigation efficiency are used to determine the quantity of irrigation water that must ultimately be supplied.

ET_c → Net Irrigation Requirement → Gross Irrigation Requirement

Supply Side

River-flow records are analysed to estimate dependable water availability during low-flow periods.

River flow record → Low-flow analysis → Dependable flow

Design principle: a canal may have sufficient hydraulic capacity to carry the required irrigation discharge, but the river may not be capable of supplying that discharge continuously during the dry season.

Crop demand

Net Irrigation Requirement

Net Irrigation Requirement (NIR) represents the depth of irrigation water that must reach the crop root zone after taking account of effective rainfall and other useful contributions to crop water supply.

NIR = ET_c − P_eff

where:

NIR = net irrigation requirement;
ET_c = crop evapotranspiration; and
P_eff = effective rainfall.

Where soil-water contribution, groundwater contribution or other effective sources are significant, these may also be included in the root-zone water balance.

System demand

Gross Irrigation Requirement

The quantity of water diverted from a river or canal must normally be greater than the net crop requirement because water is lost during conveyance, distribution and field application.

GIR = NIR / E_overall

where:

GIR = gross irrigation requirement;
NIR = net irrigation requirement; and
E_overall = overall irrigation-system efficiency.

If separate efficiencies are used, overall efficiency may be expressed as the product of the individual efficiency components.

E_overall = E_c × E_d × E_a

where E_c, E_d and E_a may represent conveyance, distribution and application efficiencies respectively, according to the terminology adopted for the particular irrigation system.

Reliable water supply

Dependable Flow

Average river discharge does not necessarily indicate the discharge that can be depended upon during the dry season. A strongly seasonal river may convey enormous quantities of water during the wet season but carry only a small fraction of this flow during the critical irrigation months.

Dependable flow therefore expresses river-water availability at a specified level of reliability.

Historical river-flow record → Low-flow statistic → Frequency analysis → Dependable flow

Meaning of a 1-in-5-Year Low Flow

P_ne = 1 / T

For a return period of:

T = 5 years

therefore:

P_ne = 1 / 5 = 0.20 = 20%

A 1-in-5-year low flow therefore has approximately a 20% annual probability of being equalled or fallen below by the selected annual low-flow statistic.

Its complementary probability is:

1 − 0.20 = 0.80 = 80%

Thus, under the same statistical definition, the selected annual low-flow magnitude has approximately an 80% probability of being exceeded.

Worked example

1-in-5-Year Low Flow Using Abbay River Data

Published monthly Abbay River flow data from Mulat and Moges (2014) provide a useful worked example. Their Table 2 gives monthly flow volumes for the historical years 1970–1978 in MCM/month.

For this demonstration, each monthly volume is first converted to mean monthly discharge:

Q = (V × 10⁶) / (D × 86,400)

where V is the monthly volume in MCM and D is the number of days in the month.

The minimum monthly mean discharge is then selected from each year. These annual minima are ranked from smallest to largest and assigned an empirical probability using the Weibull plotting position:

P_ne = m / (N + 1)

For the nine-year illustrative series:

N = 9

↔ Swipe horizontally to see all table columns.

Rank	Year	Annual Minimum Monthly Flow (m³/s)	Non-Exceedance Probability
1	1973	117.61	10%
2	1971	131.56	20%
3	1975	134.26	30%
4	1978	142.36	40%
5	1974	145.06	50%
6	1972	147.48	60%
7	1977	149.69	70%
8	1970	171.30	80%
9	1976	177.08	90%

The target probability for a 1-in-5-year low flow is:

P_ne = 20%

From the ranked annual-minimum series:

Q_{5, monthly} = 131.56 m³/s ≈ 132 m³/s

Chapter 4 — Low-Flow Frequency 1 / 2

1-in-5-Year Low Flow and Dependable Flow for Irrigation

From annual minimum flows to dependable irrigation supply

This slide shows how monthly river-flow data are converted to annual minimum flows, ranked by non-exceedance probability, and used to identify an illustrative 1-in-5-year monthly low flow of 131.56 m³/s. The critical flow is then reduced by reserved water requirements and compared with irrigation demand to determine whether the scheme has a surplus or deficit.

Design meaning: the low-flow result is not simply another discharge value. It defines a critical condition against which the irrigation scheme must be checked. If firm irrigation demand exceeds the water available under this condition, the design must provide an intentional response such as rotational scheduling, storage, improved efficiency or reduced simultaneous command area.

Source: Mulat and Moges (2014), Table 2; low-flow frequency and irrigation water-balance interpretation developed for HARE Revisited.

Design application

From Dependable River Flow to Available Irrigation Flow

The entire dependable river discharge should not automatically be assumed to be available for irrigation abstraction.

Water may need to remain in the river for environmental flow, downstream users, domestic requirements, livestock, existing abstractions and other legally or operationally reserved uses.

Q_available = Q_dependable − Q_reserve

Using the illustrative example:

Q_dependable = 131.56 m³/s

Q_reserve = 25 m³/s

Q_available = 131.56 − 25 = 106.56 m³/s

Irrigation decision

Water Balance and the Need for Irrigation Scheduling

The available dependable flow is compared directly with the irrigation requirement.

Q_balance = Q_available − Q_irrigation

Case A — Water Supply Is Sufficient

Assume:

Q_irrigation = 90 m³/s

Q_balance = 106.56 − 90 = +16.56 m³/s

Qbalance > 0

The dependable available river flow exceeds the irrigation requirement. Continuous supply is theoretically possible under the adopted assumptions.

Case B — Deficit Occurs

Assume:

Q_irrigation = 120 m³/s

Q_balance = 106.56 − 120 = −13.44 m³/s

Qbalance < 0

Continuous unrestricted irrigation cannot be maintained. Irrigation scheduling or another management response is required.

Possible responses to a negative water balance

Where dependable supply is smaller than irrigation demand, possible responses include rotational irrigation, staggered irrigation periods, adjustment of the simultaneously irrigated area, improved conveyance and application efficiency, crop-pattern modification, supplemental storage or another appropriate operating strategy.

The sign of Qbalance provides an immediate operational message:

Qbalance > 0 → dependable supply exceeds irrigation demand.

Qbalance < 0 → irrigation demand exceeds dependable supply and scheduling or another management intervention is required.

Important distinction

1-in-5-Year Low Flow Is Not Automatically the Same as Q80

The approximately 80% reliability discussed in the worked example refers specifically to the probability that the selected annual minimum monthly-flow statistic remains above the 1-in-5-year low-flow value.

A conventional Q80 derived from a flow-duration curve has a different meaning. It represents a discharge that was equalled or exceeded for approximately 80% of all selected time steps in the historical record.

1-in-5-Year Monthly Low Flow

Derived from one annual minimum monthly-flow value per year.

P(Q_min ≤ Q₅) ≈ 20%

Q80 Flow-Duration Discharge

Derived by ranking the selected flows across the complete period of record.

Q ≥ Q₈₀ for approximately 80% of observations

The two percentages may appear complementary, but the underlying statistical populations and hydrological interpretations are different.

Engineering conclusion

Dependable Flow Connects Hydrology with Irrigation Operation

Irrigation planning should not be based only on average annual river discharge or total annual water volume. The decisive question is whether sufficient water remains available during the critical irrigation period at the reliability required by the project.

Crop water requirement → Net irrigation requirement → Gross irrigation requirement → Required discharge

River record → Dependable flow → Available abstraction → Water balance

Final design question: Is dependable river-water availability sufficient to satisfy the gross irrigation requirement during the critical irrigation period?

Reference

Source Used in the Worked Example

Mulat, A. G., & Moges, S. A. (2014). Assessment of the Impact of the Grand Ethiopian Renaissance Dam on the Performance of the High Aswan Dam. Journal of Water Resource and Protection, 6(6), 583–598. DOI: 10.4236/jwarp.2014.66057. Monthly Abbay River flow data used in the worked example are from Table 2.

The 1-in-5-year calculation presented on this page is an educational frequency-analysis example developed from the published data. It is not presented as a definitive design low-flow estimate for the Abbay River. A formal project design should use the complete available long-term hydrological record.