EFFECTS OF INCREASING IRRIGATION WATER QUANTITY, AND ITS
DEPENDABILITY ON CROP YIELD.
Jonathan I. Matondo
ABSTRACT
The major purpose of water resource projects is to alter the temporal and spatial
availability of water and its reliability, to provide the necessary assurance of water when and
where needed. In reality once the system is designed constructed and put in operation (for
irrigation schemes), often farmers are not supplied with the required water demand. Instead,
water supplies become more and a random value. The objective of this study was to determine
the effects of increasing quantity or dependability or both of irrigation water supply on crop
yield. The methodology consists of an appropriate linkage of (1) a stochastic model representing
the stochastic irrigation water supply at the farm, (2) a water management strategy and constraint
for the distribution of whatever irrigation water is available to the various fields of the farm (18
acres) at each irrigation period during the growing season, (3) a model representing the soil
moisture balance model resulting from these irrigation water inputs and the anticipated
evapotranspiration of the crop concerned (wheat in this case) at the various points in time, and
finally, (4) a crop production model relating the anticipated final yield of the crop resulting from
the anticipated effects of evapo-transpiration being less than that required for optimal growth
process. The soil moisture balance model and the crop production model constitute the crop
growth simulation model. A lag one serially correlated Markov model was used as the synthetic
data generation model to generate many equally likely sequences of irrigation water supply. Each
set of equally likely sequences of irrigation water supply has a different mean and variance
combination. These values were input to the farm water management model. A farm on a
watercourse in Pakistan was used in this case study. Several on-farm water management
strategies and constraints were formulated. The appropriate on-farm water management strategy
is to irrigate the fields on a rotational basis with the conditions of 2.54cm minimum depth of
application and 60% irrigation application efficiency. The expected crop yield at 90%
exceedance probability level and a variance decrease from 25 to 0.25 for a volume of 514 m 3
water supply ranges from 12.84 to 15.1 tons and for a volume of 976.5 m 3 water supply ranges
from 16.41 to 18.32 tons. It can be concluded here that, increasing the dependability of the water
,supply (i.e., reducing its variance) was found to have a lower expected crop yield increase of
2.36 tons while increasing the mean of the water supply was found to have a higher expected
crop yield increase of 3.7 tons. Concurrent increase in the mean and dependability of the water
supply had the greatest expected crop yield increase of 5.61 tons.
Keywords: Stochastic irrigation water supply, dependability, on-farm water management
strategy, cumulative density function, expected crop yield.
INTRODUCTION1
Irrigation is used to replenish soil moisture in order to prevent the occurrence of crop soil
moisture stress which could lead to crop yield reduction. An irrigation system is designed,
constructed and operated in order to supply the required water demand. However, in reality
farmers are not supplied with the required water amount according to the design. Instead water
supplies become more and more a random value (Clyma, Kemper and Ashraf, 1977).
The physical factors that are attributed to cause a random water supply on a watercourse
in Pakistan are: leaky ditches, inadequate hydraulic capacity, thin channel walls, channels below
ground, inadequate freeboard, obstructions such as trees, weeds, debris and sediment, poor
alignment and an excessive ditch system required to serve the many small farms (Corey and
Clyma 1975). To quote Reddy (1980) “Under traditional operating conditions, the water supply
rate at the canal outlet was 3.44 cfs (97.4l/s). By simulation the field supply rate was found to be
1.83 cfs (51.8 l/s). The water was supplied to the farm on a weekly basis. The command area of
the watercourse is 515 acres (208.42 ha). Therefore, the return time is 19.59 minutes/acre/week
(48.36 minutes/ha/week). This means the farm receives water at the rate of 1.83 cfs (51.8l/s) for
19.59 minutes per acre per week”.
Figure 1 shows a comparison of the actual with the official field supply. It can be seen
from Figure 1 that, most of the time the actual supply was less than the official supply. Since the
farmer does loan and borrow water from other farmers, the times when the actual supply was less
than the official supply implies that the farmer has loaned his water to other farmers, wasted it,
or the official supply was not received.
, The times when the actual field supply was greater than the official supply indicated that
the farmer has borrowed water from another farmer or the flow was greater than the official
supply. This is a common practice under a rotational irrigation system (Clyma, Kemper and
Ashraf, 1977).
The variability of the water supply at the farm is caused by the physical factors of
watercourse water wastage and the poor management of the water distribution system.
According to Jones and Anderson (1971) the unreliability of the water supplies at critical growth
stages during each growing season has been a major constraint on the farmer in modernizing his
agricultural practices. While every irrigation system in the world is subject to fluctuations in
supplying water, some provide wider extremes than others (Jones and Anderson 1971).
According to Lowdermilk, Early and Freeman (1978), farmers in Pakistani operate under a
virtually drought situation all the time and this has been the case since the first canals were
developed. A factor which has not been evaluated is the effect of undependability of the water
supply on farmer’s practices and its effect on crop yield.
The objective of this study was to evaluate the effects of undependable (stochastic) water
supply on on-farmer water management strategies and crop yield. Specifically to determine, the
effects of increasing the quantity or dependability and or both of the water supply on crop yield.
DEPENDABILITY ON CROP YIELD.
Jonathan I. Matondo
ABSTRACT
The major purpose of water resource projects is to alter the temporal and spatial
availability of water and its reliability, to provide the necessary assurance of water when and
where needed. In reality once the system is designed constructed and put in operation (for
irrigation schemes), often farmers are not supplied with the required water demand. Instead,
water supplies become more and a random value. The objective of this study was to determine
the effects of increasing quantity or dependability or both of irrigation water supply on crop
yield. The methodology consists of an appropriate linkage of (1) a stochastic model representing
the stochastic irrigation water supply at the farm, (2) a water management strategy and constraint
for the distribution of whatever irrigation water is available to the various fields of the farm (18
acres) at each irrigation period during the growing season, (3) a model representing the soil
moisture balance model resulting from these irrigation water inputs and the anticipated
evapotranspiration of the crop concerned (wheat in this case) at the various points in time, and
finally, (4) a crop production model relating the anticipated final yield of the crop resulting from
the anticipated effects of evapo-transpiration being less than that required for optimal growth
process. The soil moisture balance model and the crop production model constitute the crop
growth simulation model. A lag one serially correlated Markov model was used as the synthetic
data generation model to generate many equally likely sequences of irrigation water supply. Each
set of equally likely sequences of irrigation water supply has a different mean and variance
combination. These values were input to the farm water management model. A farm on a
watercourse in Pakistan was used in this case study. Several on-farm water management
strategies and constraints were formulated. The appropriate on-farm water management strategy
is to irrigate the fields on a rotational basis with the conditions of 2.54cm minimum depth of
application and 60% irrigation application efficiency. The expected crop yield at 90%
exceedance probability level and a variance decrease from 25 to 0.25 for a volume of 514 m 3
water supply ranges from 12.84 to 15.1 tons and for a volume of 976.5 m 3 water supply ranges
from 16.41 to 18.32 tons. It can be concluded here that, increasing the dependability of the water
,supply (i.e., reducing its variance) was found to have a lower expected crop yield increase of
2.36 tons while increasing the mean of the water supply was found to have a higher expected
crop yield increase of 3.7 tons. Concurrent increase in the mean and dependability of the water
supply had the greatest expected crop yield increase of 5.61 tons.
Keywords: Stochastic irrigation water supply, dependability, on-farm water management
strategy, cumulative density function, expected crop yield.
INTRODUCTION1
Irrigation is used to replenish soil moisture in order to prevent the occurrence of crop soil
moisture stress which could lead to crop yield reduction. An irrigation system is designed,
constructed and operated in order to supply the required water demand. However, in reality
farmers are not supplied with the required water amount according to the design. Instead water
supplies become more and more a random value (Clyma, Kemper and Ashraf, 1977).
The physical factors that are attributed to cause a random water supply on a watercourse
in Pakistan are: leaky ditches, inadequate hydraulic capacity, thin channel walls, channels below
ground, inadequate freeboard, obstructions such as trees, weeds, debris and sediment, poor
alignment and an excessive ditch system required to serve the many small farms (Corey and
Clyma 1975). To quote Reddy (1980) “Under traditional operating conditions, the water supply
rate at the canal outlet was 3.44 cfs (97.4l/s). By simulation the field supply rate was found to be
1.83 cfs (51.8 l/s). The water was supplied to the farm on a weekly basis. The command area of
the watercourse is 515 acres (208.42 ha). Therefore, the return time is 19.59 minutes/acre/week
(48.36 minutes/ha/week). This means the farm receives water at the rate of 1.83 cfs (51.8l/s) for
19.59 minutes per acre per week”.
Figure 1 shows a comparison of the actual with the official field supply. It can be seen
from Figure 1 that, most of the time the actual supply was less than the official supply. Since the
farmer does loan and borrow water from other farmers, the times when the actual supply was less
than the official supply implies that the farmer has loaned his water to other farmers, wasted it,
or the official supply was not received.
, The times when the actual field supply was greater than the official supply indicated that
the farmer has borrowed water from another farmer or the flow was greater than the official
supply. This is a common practice under a rotational irrigation system (Clyma, Kemper and
Ashraf, 1977).
The variability of the water supply at the farm is caused by the physical factors of
watercourse water wastage and the poor management of the water distribution system.
According to Jones and Anderson (1971) the unreliability of the water supplies at critical growth
stages during each growing season has been a major constraint on the farmer in modernizing his
agricultural practices. While every irrigation system in the world is subject to fluctuations in
supplying water, some provide wider extremes than others (Jones and Anderson 1971).
According to Lowdermilk, Early and Freeman (1978), farmers in Pakistani operate under a
virtually drought situation all the time and this has been the case since the first canals were
developed. A factor which has not been evaluated is the effect of undependability of the water
supply on farmer’s practices and its effect on crop yield.
The objective of this study was to evaluate the effects of undependable (stochastic) water
supply on on-farmer water management strategies and crop yield. Specifically to determine, the
effects of increasing the quantity or dependability and or both of the water supply on crop yield.
