1. “Determination of spatially-distributed irrigation water requirements at scheme level using soil-water balance model and GIS”
Term Project – CEE 6440 – GIS in Water Resources
Tuesday, November 30th 2004
Prepared by
Daniel Zaccaria
Graduate student
Department of Irrigation Engineering
Utah State University

2. OBJECTIVES
General
Develop and test a tentative methodology for mapping I.W.R for large-scale agricultural areas, under different climatic scenarios
Evaluate spatial variability and time-distribution of I.W.R. along irrigation season
Specific
Getting a better knowledge of main factors related to irrigated agricultural systems
Identifying major sources of errors and uncertainties in large-scale estimations of irrigation requirements

3. Long-term purposes OVERALL
Carrying out preliminary studies in order to come up with a better irrigation water management plan for the study area
Carry out a performance analysis of distribution networks and eventually identify re-engineering options for low-performing systems or sub-systems
OVERALL
Show usefulness of coupling GIS environment and models capabilities to provide irrigation managers with operational tools to support decision-making processes

4. Background on the study area
The study area is part of a large-scale irrigated area located in Southern Italy and managed by a local Water Users’ Association. The whole WUA scheme covers an area of 142,905 Ha

5. The study area is served by three large-scale irrigation schemes
whose physical features and operational rules are different

6. Cropping pattern

7. Resulting consequence ENVIRONMENTAL AND ECONOMIC HAZARDS
Why this study area?
Several problems
High number of small land-holdings (average farm size ha)
Market-oriented horticulture, which is strongly depending on irrigation
Irrigation networks are operated with rotation delivery schedule
Water distribution to farms is too restrictive and is not timely matching crop water requirements
Resulting consequence
As a result of all the above factors, during the last 10 years a large number of farmers started developing their “private water sources” and refused to take water from large-scale irrigation networks.
This led to a very large number of unlicensed irrigation wells, to over-pumping from groundwater, to sea-water intrusion and salt accumulation in the soil
ENVIRONMENTAL AND ECONOMIC HAZARDS

8. ………. AND MOREOVER There is a serious water deficit in the area
An alarming message is being continuously spread out:
There is a serious water deficit in the area
But before spreading such a message we have to be able to come up with reliable numbers from application of sound methodologies

9. Therefore:
A good estimate of actual water demand is strongly needed at whole scheme level in order to identify:
Existence and potential magnitude of water deficit
Total volume of water seasonally withdrawn from groundwater
Existence and magnitude of environmental hazards
Re-engineering options (modernization and/or rehabilitation of irrigation systems) aiming at improving irrigation systems’ performances and efficiency of water use

10. Applied methodology
1st step) Identification of 3 climatic scenarios (Average, Demanding, Very-high demanding)
7 Climatic stations, monthly values for 35 years of observation ( )
Areal Clim. Deficit = SUM [(ETo – Reff) x Station Weight]
ETo computed on a montly basis by Hargreaves- Samani method
ETo = Ra (Tmax – Tmin)0.5 (Ta )
Tmax and Tmin in hot-dry situation are quite different
35 annual values of Areal Climatic Deficit
Probability of non-exceedance
50 %, 75 % and 90 Probabilities

11. Sort years and values for (Eto-P)
Region (Eto-P)
Sort years and values for (Eto-P)
Frequency (%)
1978
513.90
2.78
1993
523.74
5.56
1983
550.35
8.33
1987
559.81
50.00
1975
569.66
13.89
1968
570.84
16.67
1991
615.23
19.45
1988
628.83
22.22
1971
632.61
25.00
1994
640.35
27.78
1986
640.45
30.56
1962
644.01
33.34
1985
650.04
75.01
1977
668.03
38.89
1967
674.10
41.67
1990
676.95
44.45
1981
683.77
47.23
1961
695.21
1992
696.93
52.78
1965
714.25
94.45
1970
727.47
58.34
1989
744.94
61.12
Probability Analysis

12. Methodology
Climatic parameters => Evapotranspiration (ETo)
Crop Irrigation Requirements and their time-distribution depend on:
Crop characteristics => Crop coefficient (Kc)
Soil characteristics => WHC o AW (FC – PWP)
Soil-Water Balance => accounting for all water inflows and outflows
Di = D(i-1) + ETc – (P – SRO) – Iinf + DP - GW

13. + + + Intersect utility in ArcGIS MeteoStations polygons
Soils polygons (13 different soil types) + +
Crops polygons (8 different crops)
MeteoStations polygons +
Intersect utility in ArcGIS

14. Identification of Simulation Units
Simulation units are polygons having the same crop-soil-climatic area combinations
153 unique soil-crop-clim.area combinations = 153 ID.Codes
(153 x 3 climatic scenarios) = 459 Runnings of the Soil-Water Balance model

15. Soil-Water Balance algorithm in terms of depletion
Di = Di-1 + ETc - (P - SRO) - Irr + DP - GW
Assumptions:
irrigation aiming at maximum yield; 2) No groundwater contribution; 3) No restriction imposed
4) Soils at F.C. at irrig. season starting
Time to irrigate when:
Di > MAD Wa Rz
How much to irrigate?
The amount necessary to replenish the root-zone to the F.C.

16. Results: Map of Net Irrigation requirements

17. Superimposing layer of irrigation districts’ boundaries
Aggregation of IWR at district level for different time-scales

18. Sources of errors and uncertainties in up-scaling IWR
Spatial variability of soils
Spatial variability of crops => different crop’s age and cultivars
Spatial variability of climatic conditions (Thiessen method enables rough estimation)
Spatial variability of land elevation (not taken into account)
Spatial variability of conveyance, distribution and application efficiencies
Bottom line: I.W.R. are very close to what farmers give to crops in the area
Further check the time-distribution of seasonal IWR values

19. QUESTIONS, COMMENTS OR SUGGESTIONS ??
Aknowledgements:
G. H. HARGREAVES
Stornara & Tara Water Users Association
Technical University of Lisbon (PT) – Instituto Superior de Agricultura
QUESTIONS, COMMENTS OR SUGGESTIONS ??

20. Evaluating NIR and their spatial and time distribution
Basic information for :
Water allocation plan
Water distribution plan (time of deliveries)
Indirect quantification of withdrawals from groundwater
Investigation on available water supply and time distribution
Comparing water supply with water demand enables identification of deficit periods and areas
Performance analysis of irrigation networks based on :
Water demand flow hydrographs
Physical capability of the network
Identify critical areas
Simulate re-engineering option and evaluating their effectiveness