Strategies and
approaches for high water use efficiency of crops
towards
sustainable use of groundwater in the piedmont of
Mt. Taihang,
China
1)Hu Chunsheng*, 1)Xiying Zhang, 2)Zhao Shidong
1)Institute of Agricultural
Modernization, Chinese Academy of Sciences,
Shijiazhuang, 050021, China
2)Institute of
geographic science and natural resources, Beijing, 100101, China
*:corresponding author, Tel: 86-311-5871762,
Fax: 86-311-5815093
Email: xyzhang21@hotmail.com
Abstract
Crop yield is
greatly affected by irrigation in the plain located in front of Mt.Taihang,
China, where the rainfall is far less than the water needed by winter wheat and
summer corn as two crops a year system. Irrigation water in this area is mainly
coming from groundwater that contributes to a significant overdraft problem.
Recently, groundwater table has been decreasing at the rate of about 1m per
year. The overdraft problem has become a limiting factor to the sustainable
development of agriculture in this region. The main elements to lead water table decrease are
precipitation decreased by 130mm since 1950, expanding of wheat area which is
irrigated with groundwater, lateral flow decreased from Mt.Taihang and low WUE
of crops. The most effective way to slow the groundwater table decrease is to
increase water use efficiency of crops to reduce irrigation water use.
Many years¡¯
experiments have been carried out to study the effects of different field
management practices on water use efficiency of crops. This paper summarized
the results of optimizing irrigation schedule, straw mulching, hoeing soil
surface for preventing soil evaporation and management of root system for
efficiency water use at Luancheng Station, China. Results showed that partially
or limited irrigation schedules based on the sensitive growth stages of winter
wheat to water stress didn¡¯t reduce crop yield compared with fully irrigation
schedule, while the water use efficiency was greatly improved. By this
practice, the conventional 3 to 5 numbers of irrigation during the growing
period of winter wheat can be reduced to two or even to one number of
irrigation, a great quantity of groundwater can be saved. Critical soil
moisture contents at various growing stages for winter wheat has been decided
for irrigation scheduling. By using straw mulching, soil evaporation could be
reduced by 30%, and by hoeing soil surface to winter wheat after
over-wintering, 20mm soil water could be saved. Combining all those
water-saving measures, the water use efficiency of crops can be greatly improved.
Key
words: Water use efficiency,
Optimum irrigation scheduling, Straw mulch, Root system, Piedmont of Mt.
Taihang
Piedmont of Mt. Taihang is typical high-yield region
in North China Plain. The groundwater is basic water source for irrigation. The
groundwater overdraft has led to continuously decline of water table since 70¡¯s
and decline rate is up to about 1m per year(Table1). The fact raises the doubts
on the sustainability of agriculture based on the groundwater overdraft in this
region. How long the agricultural production of high productivity can be
maintained under the water deficit? How about the sustainable yield of water
resources for agriculture is? Where is way out to alleviate the crisis? What
measures should be taken? These questions urgently need
researchers to give definite
answer. So it is very important to study the relationship between the
agricultural development and water resources exploitation, analyze the reason
to lead decline of
groundwater
table and put forward the strategies and approaches for sustainable use of
groundwater resources.
1. The main elements affects groundwater resources
overdraft
1.1. Supplemental irrigation
is necessary for two crops of wheat and corn one year in Piedmont of Mt.Taihang.
The Piedmont of
Mt.Taihang, high-production agricultural plain, is a major grain production
region in North China Plain, China. Soil is rich and climate is favorable for
growing winter wheat and summer corn as two crops a year system in this area.
However, agricultural production in the area is limited by a lack of rainfall
during a large portion of the year and need supplemental irrigation. The
groundwater is basic water source for irrigation.
The mean annual
rainfall is about 480-500mm. The amount of rainfall fluctuates greatly from
year to year, and the distribution within a year is also very uneven. About 70%
of the total rainfall occurs during July to September, the growing season of
summer corn. The average rainfall during the wheat growing season, which is from
October to June of the following year, ranges from about 60mm to 150mm.
Supplemental irrigation is required to support wheat production because the
consumptive use of water by winter wheat is about 400 to 450mm(table 1).
Farmers in this region generally irrigate winter wheat 3 to 5 times each
season. They also irrigate corn one or two times per year(table2).
Table.
1. Water requirements by winter wheat calculated by Penmen equation recommended
by FAO in the piedmont of Mt. Taihang, China
|
Month |
Oct. |
Dec. |
Nov. |
Jan. |
Feb. |
March |
April |
May |
First ten days of June |
Total growth period (mm) |
|
Eto£¨mm/day£© |
2.2 |
1.3 |
0.9 |
0.8 |
1.2 |
2.2 |
3.6 |
4.6 |
5.1 |
566.9 |
|
Kc |
0.85 |
0.92 |
0.54 |
0.33 |
0.24 |
0.42 |
1.14 |
1.22 |
0.73 |
--- |
|
Etc£¨mm/day£© |
1.87 |
1.2 |
0.49 |
0.26 |
0.29 |
0.9 |
4.1 |
5.5 |
3.7 |
---- |
|
Etc£¨mm/month£© |
58.0 |
35.9 |
15.1 |
8.2 |
8.1 |
28.6 |
123.1 |
173.8 |
37.2 |
488.2 |
|
Rainfall£¨mm/month£© |
22.5 |
8.8 |
6.4 |
2.6 |
6.8 |
11.2 |
18.3 |
34.2 |
16.0 |
126.8 |
|
Rainfall-Etc£¨mm£© |
-35.5 |
-27.1 |
-8.7 |
-5.6 |
-1.3 |
-17.4 |
-104.8 |
-139.6 |
-21.2 |
-361.4 |
*£ºEto is reference
evapotranspiration calculated by Penmen equation using data from 1971 to 1998,
Kc is crop coefficient, Etc= Eto*Kc. Monthly rainfall was the average from 1971
to 1998.
Table.2.
Water requirements by summer corn calculated by Penmen equation recommended by
FAO in the piedmont of Mt. Taihang, China*
|
Month |
11 to 30 of June |
July |
Autumn |
1 to 20 of Sep. |
Total growth period£¨mm£© |
|
Eto£¨mm/day£© |
5.5 |
4.4 |
3.8 |
3.2 |
460.2 |
|
Kc |
0.5 |
0.81 |
1.1 |
1.07 |
--- |
|
Etc£¨mm/day£© |
2.75 |
3.56 |
4.18 |
3.42 |
--- |
|
Etc£¨mm/month£© |
55.0 |
110.4 |
129.6 |
68.4 |
363.4 |
|
Rainfall£¨mm/month£© |
40.6 |
136.5 |
119.6 |
34.9 |
331.6 |
|
Etc-rainfall£¨mm£© |
-14.4 |
+26.1 |
-10.0 |
-33.5 |
-31.8 |
*£ºEto is reference
evapotranspiration calculated by Penmen equation using data from 1971 to 1998,
Kc is crop coefficient, Etc= Eto*Kc. Monthly rainfall is the average from 1971
to 1998.
1.2.Planting area of
wheat irrigated by groundwater increasingly expand since1970's
The changes
of monthly average groundwater table from 1974-1998 (as Fig 3) show that
decline period of groundwater table is during March to Aug. which is main
growth period of winter wheat. The monthly decline rate of groundwater table
became much bigger especially in April since 1975 which means that amount of
groundwater irrigated for wheat increased (Fig4.).
The groundwater
table decreases with increasing planting area of wheat (see Fig5). The planting
area of wheat has expanded from 8000 hm2 to 25000 hm2
since 1950 (see Fig.6). The amount of water consumption for winter wheat growth
by irrigated with groundwater is important reason to lead decline of water
table.
1.3.The rainfall
decreased by about 130mm since 1950's
The climate enter
into drought and warm stage since late of 1960's and the precipitation
decreased since 1950's (see Fig7). The average rainfall is 555.42 mm in 1950's,
514 mm in 1960's, 493.96 mm in 1970's, 436.44 mm in 1980's, 420.77 mm in
1990's. The rainfall decreased by 135mm during 1950-1990 and decreased 27 mm
every 10 years. The reduced precipitation has significantly impacts on decline
rate of groundwater table (see Fig8). The relationship between precipitation
and decline rate of groundwater table express as follow formula.
Y=2.4144-0.0037X
The Y is
decline rate of groundwater table (m), the X is precipitation (mm). When
decline rate of groundwater table is zero, the precipitation is 652mm. Every
100mm rainfall reduced will lead 0.37m decline rate of groundwater table.
The effects of
reduced rainfall on the groundwater table have two aspects, one is that reduced
rainfall lead exploitation of groundwater increase and rainfall infiltration
decreased, another is that reduced rainfall lead lateral flow reduced from Mt.
Taihang.
1.4.Low lateral flow
from Mt. Taihang
The runoff enter
into the plain is decreased from 275.3 mm to 44.1 mm(see table 3), the reason
is that precipitation reduced and some runoff blocked by reservoir in mountain
area. Supposed the precipitation reduced 100mm in mountain, the human
activities lead to reduce 130mm of runoff to plain.
Table. 3. The result of water resources balance
estimation in Hebei Plain during 1950's-1980's
Year P(mm) RI(mm) RO(mm) T(
)
1950-1959
600.5 275.3
247.3
10.1
1960-1969
577.5 160.7
142.5
10.2
1970-1979
560.2 108.7 107.6
10.3
1980-1989
496.7 44.1 11.1
10.4
It is less
irrigation and rainfall season during Nov. to next Feb.. The changes of
groundwater table depends on the regional groundwater resource balance. The
ascending of water table is mainly caused by lateral flow from upriver region.
From Fig.9, the ascending rate of water table has the decreasing trend since
late of 1970's, which means the lateral flow decreased.
1.5.Low water use
efficiency of crops
Lack of
application and popularization of advanced irrigation facilities and techniques
for high WUE of crops, the WUE is low compared with developed countries.
According to the traditional management, seven times of irrigation in the
growing season of winter wheat and summer corn are needed totally, the amount of
water for each time of irrigation is about 70-80mm. The water production
efficiency for this system is only 1kg/m3, which is much lower than
2kg/m3 in some of the developed countries.
2.
The strategies
and approaches for sustainable use of grounwater resources
It involves many
aspects to establish a water-saving system towards to sustainable use of water
resources. One basic way is to reduce water exploitation such as reducing area
of crops need much water for irrigation, limiting water exploitation intensity
by suitable policies. Another is to increase WUE of crops such as implementing
advanced irrigation technologies, selecting rational irrigation schedule and
demonstrating water-saving techniques.
2.1 Selecting rational farming systems suitable to
capacity of water resources.
According to
analysis above, the winter wheat area has high correlation with the water table
decline (Fig.5). Amount of water (about300mm) used for irrigation in growing
period of winter wheat is 70% of the annual rainfall in this region. But
because there is a little rainfall in the growing period of winter wheat,
planting it more means exploring more groundwater. So taking the winter wheat
and summer corn as the principal crops of the farming system in Piedmont of Mt.
Taihang is inevitable to pose high press on the underground water resource. For
this reason, dwindling planting area of winter wheat, expanding the
drought-tolerate crops and establishing a farming system adapting to the
limited water resource is a rational choice. According to preliminary results,
if dwindling 20% of winter wheat area and applying rotation of crops
(cotton-wheat-corn), the amount of water-saving could be up to 8.9%.
2.2
Developing
comprehensive agricultural water-saving techniques and effective demonstration
model
The comprehensive
agricultural water-saving techniques are basic and effective ways to save water
resources. Its approach is to reduced water resources exploitation by
increasing reducing water waste and increasing WUE. These technologies include
irrigation schedule, tillage system such as mulching and hoeing,
drought-tolerate crop varieties, rational application of fertilizer and
irrigation ways such sprinkler, drip and tube irrigation.
2.2.1
Optimization of irrigation
schedule for high WUE of winter wheat
In the piedmont
of Mt. Taihang, winter wheat is the main irrigated crop. During its growth
period, approximately more than 300 mm irrigation water is needed for high
yield of this crop(table 1). Average yield of this crop is about 6 to 7 tons/ha.
WUE is about 13-15kg/mm.ha. Generally 3-5 numbers of irrigation are applied to
winter wheat: at over-wintering, turning green to jointing, booting to heading
and milky filling stages, respectively. For a grain crop, whose yield may
depends as much on when water is used as on the amount, and plant water
deficits do not necessarily reduce crop yields and that mild water deficits can
in fact stimulate yields (Turner, 1990). This implies that partially or limited
irrigation may not reduce crop yield. Figure 10 is the total water
consumption(ET) of winter wheat with grain yield and WUE at Luancheng station.
The results showed that the highest ET didn¡¯t produce the highest yield, and
WUE is decreasing with the increasing in ET. Then it is possible to optimize the
irrigation schedule to reduce irrigation water use, at the same time to achieve
high yield and high WUE.
2.2.1.1 Irrigation scheduling based on sensitivity stages of winter wheat
Adequate soil
moisture is essential for maximum crop production. It is well-accepted fact
that the various crop development stages possess varying sensitivity to
moisture stress(Turner, 1990). Figure 11 is the effects of water stress at its
different growth stages on the reduction of winter wheat yield, which showed
that water stress at jointing has caused the highest reduction in yield,
following is from booting to flowering, while the water deficit at turning
green and maturing had no effects on the crop yields. By using Jensen water
production function model(Jensen, 1968):
Y/Y0=P(Wa/W0)ili (1)
Where Y is the actual yield
under partial irrigtation, Y0 is the yield under non-limiting water
use from fully irrigation, n is the number of growth stages, Wa is the actual
amount of water used by the crop, W0 is the non-limiting crop water
use or potential water requirement, and li is the relative
sensitivity of crop to water stress during the ith stage of growth (sensitivity
index). The value of li for a given
crop is different at the various stages of growth. A more sensitive growth
stage has a higher value of li. The
sensitivity index of winter wheat to water stress at its different growth
period were calculated based on field results (Table 4). The highest of li appears at jointing stage.
The negative li value at
turning green stage and maturing may shows that at this two stages moderate
water stress is favorable for crop yield.
Table.
4. The sensitivity index(li) of winter wheat to water stress at its
various growth stages(Luancheng)
|
Growth stage |
Turning green to start of noding |
Jointing |
Booting |
Heading to early milky filling |
Maturing |
|
li* |
-0.1213 |
0.3145 |
0.2721 |
0.1016 |
-0.087 |
|
li** |
-0.09831 |
0.2823 |
0.201 |
0.1188 |
-0.0211 |
*:reults from
1996-1997 experiments; **:results from 1988-1989 experiments
Table.
5. The effects of irrigation
scheduling on winter wheat yield and WUE in 1996-1997(Luancheng)(seasonal
rainfall was 87.5mm)
|
Irrigation time(month-day) |
Total irrigation £¨mm£© |
Total water onsumption (mm) |
Grain field £¨kg/ha£© |
WUE £¨kg/mm.ha£© |
|
11-21 |
67.5 |
364.7 |
5500.6 |
15.08 |
|
11-21, 4-22 |
144.4 |
428.6 |
6900.8 |
16.10 |
|
11-21, 4-29 |
153.5 |
434.5 |
6164.3 |
14.19 |
|
11-21, 3-27£¬4-22 |
171.4 |
428.9 |
6494.3 |
15.14 |
|
11-21, 3-27£¬4-29 |
200.1 |
475.9 |
6308.6 |
13.26 |
|
11-21, 3-27£¬5-7 |
186.7 |
460.0 |
6503.3 |
14.14 |
|
11-21, 3-27£¬5-14 |
193.7 |
476.1 |
6219.8 |
13.06 |
|
11-21 ,4-18£¬5-14 |
194.8 |
470.1 |
7170.0 |
15.25 |
|
11-21, 4-29£¬5-22 |
176.7 |
413.2 |
6236.6 |
15.09 |
|
11-21, 3-27£¬4-22£¬5-14 |
252.5 |
474.7 |
6503.3 |
13.70 |
Table.
6. The effects of irrigation
scheduling on winter wheat yield and WUE in 1997-1998(Luancheng)(seasonal
rainfall was 126.5mm)
|
Irrigation time(month-day) |
Total irrigation£¨mm£© |
Total water consumption (mm£© |
Grain Yield £¨kg/ha£© |
WUE £¨kg/mm.ha£© |
|
|||||
|
Non irrigation |
0.0 |
299.4 |
5413.8 |
18.08 |
|||||
|
3-25£¬4-21 |
95.0 |
338.4 |
5954.9 |
17.60 |
|||||
|
3-25£¬5-20 |
151.3 |
366.0 |
5958.0 |
16.28 |
|||||
|
4-15 |
84.7 |
333.7 |
6088.2 |
18.24 |
|||||
|
3-25£¬4-21£¬5-20 |
175.9 |
375.6 |
5650.7 |
15.04 |
|||||
|
4-7£¬4-21£¬5-20 |
166.6 |
389.8 |
6066.0 |
15.56 |
|||||
Table
7 The effects of irrigation
scheduling on winter wheat yield and WUE in 1998-1999(Luancheng)(seasonal
rainfall was 60.4mm)
|
Irrigation time (month-day) |
Total irrigation £¨mm£© |
Total water consumption£¨mm£© |
Grain Yield £¨kg/ha£© |
WUE £¨kg/mm.ha£© |
|
|||||
|
Non-irrigation |
0 |
323.0 |
5325.8 |
16.49 |
|||||
|
3-16 |
80 |
366.4 |
7023.8 |
19.17 |
|||||
|
4-3 |
80 |
338.2 |
6697.5 |
19.79 |
|||||
|
4-24 |
80 |
370.4 |
7058.3 |
19.06 |
|||||
|
3-4£¬4-24 |
160 |
444.2 |
7592.0 |
17.09 |
|||||
|
3-11£¬4-24 |
160 |
438.4 |
7422.5 |
16.93 |
|||||
|
3-17£¬5-6 |
160 |
399.0 |
6915.0 |
17.33 |
|||||
|
3-17£¬5-14 |
160 |
403.9 |
7344.6 |
18.18 |
|||||
|
11-21£¬4-24 |
160 |
400.3 |
6923.0 |
17.29 |
|||||
|
3-31£¬5-5 |
160 |
442.5 |
7296.0 |
16.49 |
|||||
|
11-21£¬3-31£¬4-24£¬5-5 |
240 |
478.5 |
6937.5 |
14.51 |
|||||
Table 5, 6, 7
were the results from different irrigation scheduling in three seasons. In
1996-1997 and in 1998-1999 seasons, the rainfall was less than in normal years,
two number irrigation applied at jointing stage and booting to flowering stages
had achieved higher yield and higher WUE than the fully irrigated treatments.
In 1997-1998 season, since the rainfall was higher, a single irrigation at
jointing stage achieved the highest yield and highest WUE. The results showed
that the conventional irrigation practice in the region doesn¡¯t get highest
yield of winter wheat, and the WUE is also very lower. So it is necessary to
re-scheduling the irrigation based on the sensitivity index to water stress of
winter wheat.
2.2.1.2 The critical soil water contents at various stages of winter wheat
Results from
several studies suggest that in many situations about two-thirds of the
extractable soil water can be used before the rate of photosynthesis is
decreased(Turner, 1990). Sometime irrigation to replace water lost from soil
may be wasteful of water. This implies that when soil moisture is higher than a
certain level, decreasing of soil moisture may not reduce yield, only when soil
water contents is lower than that level, water stress will begin to cause yield
reduction.
Table
8 the critical soil moisture level (lower limit)for winter wheat at its various
growth stage(Luancheng,China)
|
Growth stage |
Seedling |
Turning green to start of
noding |
Jointing |
Booting |
Heading to early milky
filling |
Maturing |
|
Percentage over field
capacity |
60% |
55% |
65% |
60% |
60% |
50% |
Since there are
varying sensitivity to moisture stress at different growth stage of winter
wheat, there are different critical soil moisture levels. For example, at
jointing stage, the most sensitivity stage to water stress of winter wheat,
when irrigation was postponed by 7 days (soil moisture for 0-50cm decreased
from 22.5% to 17.4% by volume), yield was reduced about 11%. While at maturing,
soil moisture decreased to 16.5% by volume, no effect was found on yield in
1997. Table 7 is the list for the critical soil moisture level at various
stages of winter wheat by summing up several years¡¯ experiments at Luancheng
Station.
2.2.1.3 The optimum irrigation practice
Based on the
experimental results at Luancheng Station, the optimizing irrigation practice
in the piedmont of Mt. Taihang shall be:
(1) It is very
important that soil moisture condition at sowing is good for better germination
and emergence of winter wheat.
(2) According to soil
conditions to decide whether the irrigation before over-wintering is needed. If
the soil moisture content is over the critical level, this irrigation can be
omitted.
(3) One number of
irrigation shall be applied at jointing stage.
(4) If rainfall is
less than normal years, another irrigation shall be applied at heading to
flowering stage, otherwise, this irrigation can be omitted.
By this
irrigation scheduling, the conventional three to five number of irrigation
practice in this region can be reduced to two numbers of irrigation or even to
one number of irrigtaion, a great quantity irrigation water can be saved. Yield
of winter wheat and WUE can be increased by 10% and 15-20%, respectively.
2.2.2
Reducing soil evaporation
to increase WUE
ET is composed of
soil evaporation (E) and plant transpiration (T). WUE can be efficiently
improved by reducing soil evaporation. Figure 12 was the E and ET of winter
wheat during 1995-1996 season measured by using large-scale weighing
lysimeter(depth 2.5m, area 3m2) combining with micro-lysimeter at
Luancheng Station. The results showed that about one-third of the total ET were
E, for other crops, the percentage of E over ET was also nearly the same (table
9). In this region, for the main cropping pattern of winter wheat plus summer
corn as two crops a year system, the total E is about 250mm annually. This
value equals to three numbers of irrigation. If reducing E by 30%, WUE of crops
can be increased, and one or two numbers of irrigation can be saved. This will
be a great importance in easing the overdraft problem in the area.
Table.
9. The percentage of soil
evaporation over the total evapotranspiration for different crops(Luancheng,
1998)
|
Crops |
Total evapotranspiration(ET)£¨mm£© |
Soil evaporation(E)£¨mm£© |
E over ET £¨%£© |
|
Winter wheat |
461.8 |
137.4 |
29.8 |
|
Corn |
364.6 |
114.5 |
31.4 |
|
Cotton |
519.6 |
141.8 |
27.3 |
|
Soybean |
328.1 |
77.8 |
23.7 |
|
Millet |
319.7 |
73.1 |
22.8 |
|
Sorghum |
235.5 |
86.4 |
36.7 |
* Soil evaporation was measured by micro-lysimeter, except for winter
wheat and cotton, all other crops were planted in summer, and cotton was
planted in spring
2.2.2.1 Straw mulching in reducing soil evaporation
The piedmont of
Mt. Taihang is a high production area. Its straw sources are abundant. For a
hectare farmland, about 15-17tons of straw can be produced annually. Some of
the straw is used as composed organic manure, and others, farmers just burn
them. Results from field experiments at Luancheng Station showed that the WUE
could be improved by 10% when the summer corn is covered with wheat straw and
the winter wheat is covered by straw either from winter wheat or corn (Table
10). The 10% increase in WUE equals about 80-100mm water saved, which is about
one third of the total soil evaporation. Then it is necessary to extend this
practice to the farmland in the region.
Table.
10. The effects of straw mulching
on increasing of WUE of summer corn(Luancheng, China)
|
Year |
treatment |
Rainfall (mm) |
Irrigation (mm) |
Total water used (mm) |
Grain yield (t/ha) |
WUE (kg/ha.mm) |
Increase in WUE(%) |
|
1987 |
Mulching |
139.1 |
120 |
366.0 |
5.57 |
15.3 |
7.8 |
|
CK |
139.1 |
120 |
360.3 |
5.09 |
14.1 |
||
|
1988 |
Mulching |
343.2 |
40 |
312.1 |
4.78 |
15.3 |
10.9 |
|
CK |
343.2 |
40 |
333.5 |
4.65 |
13.8 |
||
|
1989 |
Mulching |
243.2 |
48 |
321.8 |
6.52 |
20.3 |
10.7 |
|
CK |
243.2 |
48 |
345.1 |
6.32 |
18.3 |
||
|
1990 |
Mulching |
393.4 |
0 |
326.0 |
6.32 |
19.4 |
9.3 |
|
CK |
393.4 |
0 |
342.6 |
6.00 |
17.6 |
||
|
1992 |
Mulching |
210.3 |
140 |
342.0 |
6.33 |
18.5 |
6.0 |
|
CK |
210.3 |
140 |
350.4 |
6.12 |
17.4 |
For straw
mulching practices, it is relatively easier to cover the summer corn with the
wheat straw, since farmers usually plant maize to the wheat field ahead of 5 to
10days before harvesting of wheat. And combine harvester is widely used. After
harvesting, farmers just spread the straw evenly in the field. But for covering
the winter wheat using straw, it is relatively labor costing. Generally when
the winter wheat grows three leaves, the straw which has been cut up is spread
between rows. The wind will not blow off the straw since the existence of the
wheat seedlings. Besides, the straw mulching practice can also increase soil
organic contents.
2.2.2.2 Reducing soil evaporation by hoeing soil surface
Hoeing is a traditional
practice in history in China for wiping out weeds and reducing soil
evaporation. By hoeing soil surface, water transfer to soil surface can be cut
off. Especially, when the winter is over, soil begins thawing, and soil water
flows from deeper layer of soil to soil surface, hoeing lightly to the soil
surface between rows of winter wheat can reduce soil evaporation, since at this
time the leaf area index of winter wheat is very lower. Figure 13 compared of
soil water contents with and without hoeing. The results showed that the water
content of hoed soil was higher than the one without hoeing. By calculation,
From late of February to the beginning of April, about 20mm evaporation from
soil surface could be reduced.
2.2.3
Managing root system for
reducing irrigation by deep tillage
The amount of
water available to a crop depends both on the amount of water stored in soil
profile at sowing and on rainfall and irrigation during the life of the crop
(Passioura, 1983). The role of roots in miniming both water evaporation
directly from soil during the growth season, and that left behind in the soil
at the end of the growth season has important implication in increasing WUE and
yield for limited water supply of crops.
After irrigation
or rainfall, the water uptake activity of roots at soil surface will influence
the proportion of the water supplied that is extracted by roots before it has
been evaporated directly from soil. Generally, root length density at top layer
of soil is much higher than that at deep layer of soil profile. This
distribution of root system is favorable for root extracting water at soil
surface, that is why sometime crop can utilize soil water below wilting point
at top layer of soil, while substantial water at deep layer of soil still left
at harvest, even to water limited crops. Figure 14 shows the distribution of
soil moisture at sowing and harvest for non-irrigated winter wheat. Though the
winter wheat had been affected by water stress, there was still some available
water stored at deep layer of soil, while at top layer of soil, the water
contents was nearly the same with wilting point. The water uptake pattern of
crops is closely related to their root distribution along soil profile.
Availability of
soil water to plants depends on how fast the roots are extracting the water.
The rate of this extracting from a given volume of soil is proportional to
rooting density, diffusivity of soil water and the water potential gradient
between soil and root(Tinker, 1976). When soil water content is not the
limiting factor, root length density (RLD) will be the main factor influencing
soil water utilization rate
for a given soil layer. Several years¡¯ experiments at Luancheng Station showed
that when RLD is lower than 0.8cm/cm3 for winter wheat, the root
will be the main factor limiting soil water use.(Zhang and Yuan, 1995).
Figure 15 shows
the distribution of root system at harvest for winter wheat and maize in the
piedmont. The average maximum root depth of winter wheat can reach 2m, for
maize, it is about 1-1.2m because of its relative short growth period.
Generall, the RLD of winter wheat below 80cm of soil surface is less than
0.8cm/cm3. In this high production plain, water contents of the 2m
root zone is near field capacity because of the rainy season in July, August
and September at sowing. And at harvest, a substantial available soil water
(generally about 100 to 150mm) stored at soil layer below 1m hasn¡¯t been
utilized. Then if improving the root growth at deep layer of soil, more stored
soil water will be utilized by winter wheat, irrigation water may be reduced.
For increasing
root growth at deep layer of soil, one effective method is to break the soil
pan by deep tillage (Barracloghu and Weir, 1988). Figure 16 shows that the root
growth of winter wheat in deep layer of soil was greatly improved by deep
tillage( plough depth was 50cm) than that the conventional tillage(plough depth
about 30cm) and yield was improved by 10% to the one number of irrigation
treatment for the dry season of 1998-1999. More soil water was used from deep
layer of soil..
2.2.4
Implementing the
relatively advanced technologies like sprinkling and driping irrigation to
reduce water waste as transfer and increase WUE
The different
irrigation ways have different water transfer efficiency and different effects
on water-saving. The results from Tab.11 show that spray irrigation could
greatly reduce water amount of irrigation and increase WUE. The spray
irrigation could increase WUE by 20% than that of the furrow irrigation. Agricultural
scientists should demonstrate these advanced irrigation ways to farmers and
government should increase input for improving irrigation facilities to replace the traditional method like
the overflowing irrigation
Table 11
Effects of irrigation ways on the yield and water use efficiency of
winter wheat*
Treatments
Rainfall Irrigation Consumption of Total water use yield WUE
(mm)
(mm) soil water
(mm) (mm) (kg/hm2) (kg/hm2.mm)
Spray irrigation 93.6 140.0 170.8 404.4 7812 19.35
Pipe irrigation 93.6 170.0 168.2 435.8 7953 18.30
Furrow irrigation 93.6 241.6 138.8 474.0 7608 16.05
*by Chen Suying.
2.3
Making the
suitable policies to control the over use of the water resources
Water price is
one of key elements effecting the implementation of the results. It will
provide very positive influences on the water conservation and sustainable to
increase the water price to control the over-exploration of the underground
water resource. So local governments should develop new relative policies to
ensure sustainable use of water resources.
3.
Summary
The yield of a
crop is the outcome of myriad process occurring at many time scale((Passioura,
1976). And in many crops the extractable soil water content can be reduced by
50% before there is any influence on physiological activity leading to loss of
crop productivity(Turner, 1990). Results showed that irrigation can be reduced
and mild water deficits will not reduce winter wheat yield at some stages in
the piedmont of Mt. Taihang in China. The critical soil water contents and the
sensitivity to water stress at the different water growth stages of winter
wheat can be used as indicators for irrigation scheduling. Also by improving
deep root growth, more soil water stored during the rainy season before sowing
can be utilized by winter wheat for the purpose of reducing irrigation water
use. Other effective methods such as straw mulching and hoeing soil surface can
improve WUE by reducing soil evaporation. Combining all those measures, the
water use efficiency of crops can be improved. This has an important
implication in sustainable utilization of the groundwater resource in the
region.
For farmers to
adapt those water-saving management practices, public policies are very
important. Presently, farmers in this region only pay the electricity cost in
drawing groundwater for irrigation. To control the descending of groundwater
table, besides the field management practices, public policies, such as
imposing a per-unit tax on use of groundwater, raising electricity price for
pumping when water use per field area exceeds a specified amount, should also
be used to promote farmers to use water-saving measures to increase WUE and to
reduce the demand for irrigation water.
Acknowledgements
This project is
jointly supported by Key Projects of Chinese Academy of sciences (KZ95T-04-01
and KZ951-A1-301), National Scientific Program of 96-006-02-03-3 and the
cooperation research project of LWR!/95/07 founded by ACIAR of Australia.
References
Barraclough PB, Weir,
AH,1988. Effects of a compacted subsoil layer on root and shoot growth, water
use and nutrient uptake of winter wheat, J.
Agri. Sci. Cam., 110:207-216
Carl C, 1993. Use of
microlysimeters to measure evaporation from sandy soils, Agri. and For. Mot., 65:159-173.
Chen Suying et al., 1998,
Effect of comprehensive water-saving techniques on water wheat in Taihang
Piedmont, Chinese Eco-Agriculture
Research,6(2):61-63.
Doorenboss J, Pruitt WO,
1977. Crop water requirements, FAO Irrigation and Drainage Paper, 24,
Rome.
Doorenboss J, Kanssan, A,.
1979. Yield response to water, FAO
irrigation and drainage paper, 33 Rome, 1979, p193.
English MJ, Nakamura B,
1989. Effects of deficit irrigation and irrigation frequency on wheat yields, J. Irri. Drain. Eng., ASCE, 115:172-184.
Ghahraman B, Sepaskhah AR,
1997. Use of a water deficit sensitivity index for partial irrigation
scheduling of wheat and barley, Irri.
Sci., 18:11-16.
Hu Chunsheng et al.,
modeling dynamics of groundwater in Luancheng County, Progress in Geography,
17(supp.): 26-31.
Hu Chunsheng et al., An
analysis on structure succession of farmland ecosystems and its function changes
in Luancheng County, Chinese
Eco-Agriculture Research, 6(2):51-54.
Jensen ME, 1968. Water
consumption by agricultural plants, in: Kozlowski TT(ed.), Water deficit and plant growth, Vol.2, Academic Press, New York,
pp1-22.
Liu CM, Zhang XY and You MZ,
1998. Determination of daily evaporation and evapotranspiration of winter wheat
field by large-scale weighing lysimeter and micro-lysimeter, J. of Hydrology, 10:36-39 (in Chinese).
Passioura JB, 1983. Root and
drought resistance, Agric. Water Mana.,
7:265-280.
Zhang XY, Yang XL, 1995.
Afield study on the relationship of soil water content and water uptake by
winter wheat root system, ACTA
Agriculturae Boreali-Sinica, 10(4):99-104 (in Chinese).