Engineering comparison

1995 Study vs Modern Models

The purpose of the modern reassessment is not to replace or erase the original 1995 HARE Irrigation Project analysis. The historical design is preserved as the engineering baseline, while HEC-HMS, SWAT and CROPWAT provide independent modern checks using updated datasets, GIS, hydrological modelling and FAO crop-water procedures.

1995 design

Historical baseline

Original hydrology, irrigation demand, canal design, hydraulic structures and diversion-headworks calculations.

HEC-HMS

Flood hydrology

Design rainfall, rainfall losses, hydrographs and peak flood discharge for headworks verification.

SWAT

Watershed balance

Rainfall, evapotranspiration, runoff, groundwater, lateral flow and long-term watershed behaviour.

CROPWAT

Irrigation demand

Reference ETo, crop evapotranspiration, effective rainfall and irrigation-water requirement.

Comparison principle. Agreement between the historical and modern results strengthens confidence in the original design. Differences are investigated in terms of climate period, data source, spatial resolution, modelling method and engineering assumptions rather than being treated automatically as errors.

Flood hydrology

Design Peak Flood Comparison

The original 1995 flood estimates are retained as the historical design benchmark. The modern HEC-HMS model is then used to independently derive event hydrographs and peak discharges for selected return periods. SWAT is used principally as a longer-term watershed model rather than as the primary design-flood model.

Return period	1995 peak Q (m³/s)	HEC-HMS (m³/s)	SWAT / other check	Interpretation
5 years	25.289	Pending	SWAT is retained as a watershed-scale hydrological consistency check rather than the principal event-flood estimate.	Modern HEC-HMS result will be added after completion and checking of the corresponding design storm.
10 years	33.372	Pending	SWAT is retained as a watershed-scale hydrological consistency check rather than the principal event-flood estimate.	Modern HEC-HMS result will be added after completion and checking of the corresponding design storm.
25 years	46.655	97.0	SWAT is retained as a watershed-scale hydrological consistency check rather than the principal event-flood estimate.	The modern HEC-HMS simulation produced a peak discharge of approximately 97 m³/s for the 25-year event.
50 years	59.183	114.0	SWAT is retained as a watershed-scale hydrological consistency check rather than the principal event-flood estimate.	The modern HEC-HMS simulation produced a peak discharge of approximately 114 m³/s for the 50-year event.
100 years	74.397	Pending	SWAT is retained as a watershed-scale hydrological consistency check rather than the principal event-flood estimate.	Modern HEC-HMS result will be added after completion and checking of the corresponding design storm.

Hydrological interpretation. HEC-HMS provides the principal modern design-flood check because it is configured for event-based rainfall-runoff modelling. SWAT complements this by testing whether the rainfall, evapotranspiration, runoff and groundwater behaviour are hydrologically realistic over longer periods.

Watershed water balance

SWAT Hydrological Check

The SWAT simulation provides an independent view of the long-term water balance of the Hare watershed. These values are annual averages from the simulation period and should not be interpreted as single-year totals.

SWAT parameter	Simulation result	Engineering meaning
Mean annual precipitation	1665.5 mm	Average annual climatic input over the SWAT simulation period.
Potential ET	1570.7 mm	Atmospheric evaporative demand estimated by the selected SWAT PET method.
Actual ET	928.9 mm	Simulated evapotranspiration actually realised by the watershed.
Surface runoff	267.63 mm	Direct surface contribution to watershed discharge.
Lateral flow	250.62 mm	Subsurface lateral water movement toward the stream network.
Groundwater return flow	182.06 mm	Simulated groundwater contribution to streamflow.
Average curve number	77.62	Represents the average runoff response of the simulated watershed.

Crop water demand

1995 Crop-Water Method vs Modern CROPWAT

The crop-water comparison is methodologically important because the original 1995 Hare study used the FAO procedures available at the time, while the reconstruction uses modern CROPWAT procedures. The physical basis remains similar: reference evapotranspiration is converted into crop evapotranspiration using crop coefficients, effective rainfall is deducted, and irrigation losses are added to determine diversion demand.

Component	Hare 1995	Modern CROPWAT reconstruction	Comparison objective
Reference ET	Modified Penman method	FAO Penman–Monteith procedure	Compare annual magnitude and monthly seasonality.
Crop coefficient	Kc values derived for crop-development stages	Crop-specific Kc values entered in CROPWAT	Check whether modern crop coefficients alter ETc.
Crop evapotranspiration	ETc derived from ETo × Kc	ETc = Kc × ETo	Direct crop-by-crop comparison.
Effective rainfall	Effective rainfall and storage-factor procedure	CROPWAT effective-rainfall calculation	Determine how much rainfall contributes to crop demand.
Net irrigation requirement	NIR = ETc − Pe	NIR calculated from ETc and effective rainfall	Compare field-level irrigation requirement.
Conveyance efficiency	80%	To be tested against reconstructed canal condition	Determine effect of canal losses on diversion requirement.
Application efficiency	70%	To be reviewed	Compare field-application assumptions.
Field canal efficiency	80%	To be reviewed	Assess distribution losses.
ETc adjustment	20% reduction adopted in the 1995 design because of the humid environment associated with nearby lakes and vegetation	No automatic reduction; modern climate data are used directly	Test whether the historical adjustment remains justified.
Final output	Monthly water requirement and diversion discharge	ETc, effective rainfall, NIR, GIR, irrigation schedule and scheme discharge	Compare modern scheme demand directly with the historical design.

Climate and evapotranspiration

FAO Climate Reference vs SWAT

Parameter	FAO historic climate	SWAT simulation	Interpretation
Climate period	1961–1990	Recent SWAT simulation period	Different periods; direct rainfall equality is not expected.
Annual precipitation	811 mm	1665.5 mm	Difference reflects climate period, data source and spatial representation.
Annual ETo / PET	1622 mm	1570.7 mm	Difference is approximately 3.2%, indicating close agreement in atmospheric evaporative demand.

Important. FAO CROPWAT ETo and SWAT PET are not automatically identical quantities. Their close annual magnitudes are used here as a consistency check. The selected PET method in SWAT should be documented when the final technical comparison is prepared.

Current status

Comparison Programme

Completed

HEC-HMS 25-year event

97 m³/s

Modern design-flood simulation completed.

Completed

HEC-HMS 50-year event

114 m³/s

Modern design-flood simulation completed.

Completed

SWAT water balance

1665.5 mm

Mean annual simulated precipitation with water-balance components available.

In progress

CROPWAT crop demand

Next stage

Crop-by-crop water requirements and irrigation demand will be added as runs are completed.