Model data

Climate & ETo

Climate data provide the atmospheric forcing required for crop-water analysis and watershed simulation. The HARE reconstruction retains the historical FAO climate reference used for CROPWAT analysis while also comparing it with the more recent climate representation used in SWAT.

Important. Climate datasets from different periods and sources should not be expected to reproduce identical rainfall totals. Their role is to provide documented climatic inputs appropriate to each model and to allow transparent comparison of rainfall and evaporative demand.

FAO climate reference

1961–1990

Historical climate normal used as the reference climate dataset for the CROPWAT reconstruction.

Reference location

6.03°N, 37.56°E

Approximate Arba Minch / Hare reference location, elevation about 1239 m.

Annual rainfall

811 mm

Long-term FAO historical annual rainfall total.

Annual reference ETo

1622 mm

Annual reference evapotranspiration from the FAO climate dataset.

Historical climate

Monthly FAO Climate Data

The following monthly climate values are used as the historical climate reference for the modern CROPWAT assessment.

Month	Rainfall mm	Wet days	Tmin °C	Tmax °C	Tmean °C	RH %	Sunshine %	Wind m/s	ETo mm/month	ETo mm/day
January	24	2.7	13.8	31.0	22.4	41.5	80.2	1.2	145	4.7
February	22	3.6	15.0	31.6	23.3	41.6	80.7	1.2	142	5.1
March	70	5.5	16.0	31.4	23.7	47.8	73.2	1.6	168	5.4
April	135	11.3	16.5	29.9	23.2	59.4	63.9	1.3	141	4.7
May	128	9.9	16.7	28.3	22.5	66.0	64.4	1.3	135	4.4
June	53	8.6	16.4	27.4	21.9	64.7	59.9	1.3	123	4.1
July	42	7.8	16.2	26.4	21.3	65.6	51.6	1.3	119	3.8
August	46	8.4	16.2	26.9	21.5	65.1	56.9	1.3	127	4.1
September	94	7.4	16.1	28.1	22.1	62.1	64.5	1.2	132	4.4
October	116	6.8	15.6	28.6	22.1	64.4	64.7	1.2	132	4.3
November	60	5.7	14.0	29.3	21.6	58.5	72.4	1.0	125	4.2
December	21	3.4	13.0	30.1	21.5	49.7	82.3	1.0	133	4.3
Annual / mean	811	81.1	—	—	—	—	—	—	1622	—

Modern watershed check

FAO Climate vs SWAT

The SWAT simulation provides a useful independent comparison of precipitation and atmospheric evaporative demand. The comparison must be interpreted carefully because the two datasets represent different periods and modelling procedures.

Parameter	FAO historic climate	SWAT simulation	Interpretation
Climate period	1961–1990	Modern SWAT simulation period	Periods differ and therefore direct rainfall equality is not expected.
Annual precipitation	811 mm	1665.5 mm	Difference may reflect period, climate source, spatial averaging and model representation.
Reference ETo / PET	1622 mm	1570.7 mm	Annual values are relatively close, differing by about 3.2%. They are useful as a consistency check but are not automatically the same physical model variable.
Actual ET	—	928.9 mm	SWAT actual ET represents simulated water actually lost through evaporation and plant transpiration.

Interpretation. The similarity between annual FAO reference ETo and SWAT PET provides a useful broad-scale check of atmospheric evaporative demand. However, the final HARE documentation should record the PET method selected in SWAT before making a detailed method-to-method comparison.

Model use

How Climate Data Feed the Models

HEC-HMS

Design Rainfall

HEC-HMS primarily uses rainfall-event data developed from daily rainfall, frequency analysis and design-storm construction. Monthly climate averages are not the principal event input.

SWAT

Continuous Climate

SWAT uses continuous weather series to simulate rainfall, evapotranspiration, runoff, soil water, groundwater and streamflow over long simulation periods.

CROPWAT

Reference ETo

CROPWAT uses monthly climate variables to calculate or define reference evapotranspiration, which is combined with crop coefficients to determine crop evapotranspiration and irrigation demand.

Data status

Climate Data Status

Dataset	Use	Status
FAO historical monthly climate	CROPWAT reference climate and historical comparison.	Available
FAO monthly reference ETo	CROPWAT crop-water calculations.	Available
SWAT daily climate	Continuous watershed simulation.	Available
SWAT PET method documentation	Required for final ETo/PET comparison.	Document method