Tải bản đầy đủ - 0 (trang)
II. Summer Fallow: A Second Look

II. Summer Fallow: A Second Look

Tải bản đầy đủ - 0trang

Table I

Soil-Water Storage (SWS)and Precipitation Storage Efficiency (PSE) during Specific Periods of the 21-Month Fallow in a Spring Wheat-Fallow System

Specific periods of the 21-month fallow"

Harvest to spring



Spring to fall



SWS

(mm)



PSE

(70)



of

total

SWS



SWS



PSE



gr, of

total



(mm)



(%)



60



33



54



51



83



60



76



72

80



51



56



65

62



o/c



Reference



Haas and Willis, 1962

Plow (Mandan, ND)

Black and Power, 1965

Minimum- and no-tiU (Sidney, h4T)

Tanaka and Aase, 1987

Stubble-mulch (Sidney, MT)

No-till



Yearsofdata



8 of

total

SWS



PSE



SWS



SWS

(mm)



17



46



0



0



0



111



19



10



5



9



16



19



15



109



27



34

41



19

23



31

31



4

9



9

19



4

7



110

130



29

35



(%)



SWS

(mm)



PSE



(a)



1915-1954

1956-1964

1981-1 985



Mean precipitation (mm)

Soil water storage (mm)

Precipitation lost (mm)

"SWS (for a given fallow period)



Entk21-mth

fallow



Fall to seeding



15 1



74

77

= profile



214

34

180



64

7

56



429

I15

3 14



soil water at the end minus the profile soil water at the beginning of the fallow period. PSE (for a given fallow period)



= (SWSdivided by precipitation during that fallow period) X 100. Percentage of total SWS = SWS during a given period of fallow divided by total stored water during the entire fallow) X 100.



DRYLAND CROPPING INTENSIFICATION



203



mm of summer precipitation provides the potential for 288 mm of available water

for possible inclusion of a summer crop in the rotation.



B. WINTERWHEAT-FALLOW

SYSTEM

Black et ul. (1974) summarized 14 years of winter wheat-fallow data from Sidney, Montana, and divided the fallow into two periods: (1) harvest to spring, and

(2) summer of fallow. For stubble-mulch fallow, an astonishing 84% of the total

fallow storage was saved from harvest to spring, a period in which only 36% of

total fallow precipitation was received. The remaining 64% of precipitation (216

mm) during the summer of fallow only contributed 16% (15 mm) to storage and

the rest (20 I mm) to evaporation. Greb et ul. ( 1967) studied the effects of mulch

loading rates on fallow storage in a winter wheat-fallow system and reported that

water storage from harvest to late spring represented over 90% of the total fallow

storage (determined from Table I1 by summing storage from harvest to fall and

from fall to late spring). As shown in Table 11, a large portion of precipitation storage occurred during winter of fallow. During the summer of fallow (from late

spring to seeding), 7 out of 10 experiments yielded a negative water storage, even

under residue amounts as high as 10 t ha-'. Over the entire fallow period, fallow

storage increased with increasing residue loading rate. Examining a range of

tillage and residue management methods, the work of Smika and Wicks (1968)

and Tanaka and Aase (1987) confirmed previous findings (Table 11).

The most intriguing observation from Table I1 is that across the Great Plains,

from 68 to 148 percent of total precipitation storage in the entire 14-month fallow

period was achieved from harvest to spring. The overwinter period was by far the

most efficient, and the summer of fallow the least efficient period of fallow. Precipitation during the latter period was almost entirely lost to evaporation. The problem of low precipitation-storage efficiency has been only partially improved by

modem tillage and residue management practices. Fallow storage efficiency has

increased from the 10% range under intense tillage operation at the turn of this

century to the 20-30% range under the modern no-till and residue management

techniques. It is evident that even under modem conservation practices, the original criticism of fallow still remains, and fallow precipitation-storage efficiency remains low.



In. DRYLAND CROPPING INTENSIFICATION

Enhanced soil and water conservation is essential to the sustainability of dryland agriculture in the Great Plains. Fallowing is highly inefficient, as shown in



Table I1

Soil Water Storage (SWS)and Precipitation Storage Efficiency (PSE) during Specific Periods of the 14-Month Fallow in a Winter Wheat-Fallow System

Specific periods of the 14-month fallow”



Harvest to late fall



Reference

Greb et al., 1967

Sidney, MT



Akron, CO



North Platte, NE



Smika and Wicks, 1968

Plow (North Platte, NE)

Stubble-mulch

Reduced-till

No-till

Tanaka and Aase, 1987

Stubble-mulch (Sidney,MT)

No-till



Residue

level

(ths-’)



Late fall to late spring

% of



Yearsofdata



SWS

(mm)



PSE



Entire 14-month fallow



% of



SWS



PSE



(mm)



(%)



-



-



56

67



-22

-21

-7

15

18

20

-7

-3

-17



12



178

184

220

258



64

66

78

92



122

91

97

94



37

35



41

38



52

48



41

33



total

SWS



SWS

(mm)



PSE



9

5

5

28

19

24

48

45

40



78

87

86

87

114

119

105

114

150



-



-30

2



34



-23

2

6

18



37

40



43

46



(%)



Late spring

to seeding



(%)



total

SWS



Precip.

(mm)



SWS

(mm)



PSE



-



355

355

355

549

549

549

648

648

648



56

66

79

142

163

183

190

203

223



16

19

22

26

30

33

29

31

34



12

14

-6

-18



7

9

-4

-11



640

640

640

640



146

203

226

214



23

32

35

43



21

36



16

27



299

299



99

114



33

38



(%)



1962- 1965

0

1.7

3.4

1.7

3.4

6.7

3.4

6.7

10.1



5



3

4

40

31



-



-



4 4 92

92

90

-



-



-



139

132

109



61

70

65

55



-



-



-



1963-1966

-44

4

11



5



1981-1984



“SWS (for a given fallow period) = profile soil water at the end minus the profile soil water at the beginning of the fallow period. PSE (for a given fallow period)

= (SWSdivided by precipitation during that fallow period) X 100. Percentage of total SWS = (SWS during a given period of fallow divided by total stored water during the entire fallow) X 100.



DRYLAND CROPPING INTENSIFICATION



205



the previous section. The soil-water storage data suggest that enhanced efficient

use of precipitation may be possible if summer crops are inserted in periods that

have low water-storage efficiency.

In the remainder of this chapter, we use data from the Sustainable Dryland Agroecosystem Management Project (Peterson et al., 1993) as a case study to develop

a better understanding of the concept of intensification and its influence on precipitation storage and use. That project was established in 1985 to address precipitation use efficiency under dryland-no-till cropping systems at three locations in

the west-central Great Plains region. The experimental locations, with long-term

precipitation ranging from 400 to 450 mm year-', represent nearly a two-fold increase in pan evaporation from north (Sterling, Colorado) to south (Walsh, Colorado). The crop-management systems imposed in each location are a continuum

with increasing cropping intensity and fewer summer fallow periods per unit time

(Table 111).All systems are managed with no-till techniques. The benchmark cropping system is the winter wheat-fallow (WF). Cropping intensity increases for the

3-year rotations of winter wheat-corn-fallow (WCF) and winter wheat-sorghumfallow (WSF), and the 4-year rotations of winter wheat-corn-millet-fallow

(WCMF) and winter wheat-sorghum-sorghum-fallow (WSSF). Herafter we refer

to sorghum in WSF and the first-year sorghum in WSSF as sorghum- 1 and the second-year sorghum in WSSF as sorghum-2.

In part A of this section, we provide a summary of results tr7nn-rkepreceding

case study along with other modern no-till cropping studies from the central and

southern Great Plains region. Our intention is to reiterate the state-of-the-art research findings regarding the potential of intensifying cropping systems in the

Great P1ai n s .



A. MODERNDRYLAND-NO-TILL

CROPPING

SYSTEMS

Table IV provides a summary comparison of modern no-till winter WF and

more intense 3- and 4-year cropping systems from the Great Plains. The longest

fallow period in a dryland cropping system always precedes the winter wheat crop

and varies in duration from approximately 14 months in WF to 10-13 months in

the 3- and 4-year systems. Length and time of fallow influence the amount of precipitation received during fallow, with the 14-month fallow in WF having the

largest mean precipitation of 657 mm for all locations.

Two of the most significant observations from Table IV are as follows. First,

available soil water at wheat planting in all systems at a given location was similar, in spite of the fact that precipitation received during the 14-month fallow in

WF was 140-250 mm greater than precipitation during the fallow preceding wheat

in the 3- and 4-year systems. We can conclude that available soil water at wheat

planting is not a function of the intensity of the cropping system as long as the



Table III

Yearly Crop and Fallow Sequence in the Dryland Cropping Systems Case Study at the Sterling and Stratton [wheat-fallow (WF),

wheat-corn-fallow (WCF), and wheat-corn-millet-fallow(WCMF)] and Walsh [wheat-fallow (WF), wheat-sorghum-fallow (WSF),

and wheat-sorghum-sorghum-fallow(WSSF)] Experimental Locationsin Eastern Colorada"

Cropping

system

WF

WF

WCF, WSF

WCF, WSF

WCF, WSF

WCMF, WSSF

WCMF, WSSF

WCMF, WSSF

WCMF, WSSF



Yearly crop and fallow sequence

1988-1989



1989-1 990



1990-1991



1991-1 992



1992-1993



1993-1994



W

F

W



F

W



W

F

F

W



F

W

W



W

F



F

W



-C(S)



- C(S)

F



F

W

-C(SI)

-M(S2)

F



W

-C(Sl)

-M (S2)

F

W



- C(S)

- M(S2)



F

W

-C(Sl)



- C(S)



-C(S)

F



F

F

W

-C(Sl)

-M(S2)



W

W

-C(Sl)

-M(S2)

F



W

F

W



F

W

-C(S)

-C(Sl)

-M(S2)

F

W

~~



1994-1 995



- C(S)

F

-M(S2)

F

W

-C(Sl)



~~



"W, winter wheat; F. fallow preceding wheat; C(S), corn (sorghum); C(S 1). corn (first sorghum following wheat); M(S2), millet (second sorghum following wheat).

At Walsh, proso millet (1989-1990) and forage sorghum (1 991-1992) were planted in place of second sorghum in the WSSF system. All phases of each cropping system are present every year.



DRYLAND CROPPING INTENSIFICATION



207



wheat is preceded by a lengthy fallow. One may additionally conclude that from a

precipitation management perspective, the similarity of soil water at wheat planting favors the intensified systems over WF, since on the average 200 mm less precipitation was required in the intensified systems to store nearly the same amount

of soil water. Even under the most intense conservation practice of no-till, the

amount of precipitation stored in the soil profile during the lengthy fallow that precedes winter wheat is extremely low, ranging from a minimum of 11% at Bushland, Texas, to a maximum of 27% at Stratton, Colorado.

The noncrop (fallow) period preceding corn (sorghum- 1) plantings averages 11

months. As shown in Table IV (and previously by McGee et al. [ 1997]), both precipitation storage and efficiency during the noncrop period preceding corn

(sorghum-1) were much greater than for fallow preceding wheat in WF, even

though precipitation was on average 232 mm less in the former than in the latter

case. For instance, mean precipitation storage and efficiency during the noncrop

period preceding corn (sorghum-1) were 137 mm and 35% storage, respectively,

compared with 119 mm and 18% storage for WF.

The second important observation (Table IV) is that at each location, plantavailable soil water at corn (in WCF and WCMF) and sorghum-1 (in WSF and

WSSF) planting was very similar to the corresponding soil water at wheat planting, in spite of the fact that corn (sorghum-1) was planted in May (June) while

wheat was planted 5 (4) months later in September. This was true even though

nearly 6 0 4 5 % of annual precipitation occurred during the 4-5 month period between corn (sorghum-1) and wheat planting. This means that most precipitation

received during the summer of fallow just preceding wheat planting in the WF system was lost. That precipitation, however, was efficiently used in the intensified

systems to grow an additional crop of corn or sorghum. Having equal amounts of

soil water at wheat and corn (sorghum-1) planting reflects the underlying basis for

the practice of opportunity cropping, in which spring soil-water content is used to

determine if conditions are favorable for summer cropping. According to soil-water profile values at corn (sorghum- 1) planting (Table IV), spring conditions for

summer cropping were on average favorable at all locations.



B. SUMMERCROPPING:

KEYTO EFFICIENT

USE

OF PRECIPITATION

In the preceding section, we document the potential for intensifying the 2-year

WF system. Here we investigate the key elements influencing the efficient use of

precipitation by cropping intensification and begin by examining precipitation

storage and efficiency during different periods of the long fallow preceding wheat.

Figure 1 illustrates a side-by-side comparison of the WF and WCF systems. The

long fallow in WF can be divided into three distinct periods: (1) early period (from



Table IV

Summary of Plant-Available Soil Water (PASW) at Crop Plantingand Precipitation (P), Soil-WaterStorage (SWS),

and Precipitation Storage Efficiency (PSE)during the Noncmp (Fallow)Period Just Preceding That Cmf



Unger. 1994



1984-1991



Bushland. TX

(notlll)



Nowood, 1994



WSF



226



463



74



16



228



598

455



I37



23



-



-



-



146



32



215



38 I



175



517



I45



1987-1992



Ga&n City. KS

noti ill)



Jones and Popham. 19Y7



WF



212



WSF



181



WF



212



730



80



II



-



-



-



WSF



205



477



81



17



214



480



101



1984- I993



Bushland. TX

(no-1111)



28



-



-



-



-



Farahani el 0 1 . 1998

Sterling. CO (no-till)



Stranon. CO (no till)



Walah. CO (no-ull)



Mean



198&199S



WF



I a7



610



99



16



-



-



-



-



WCF



I76



420



82



19



I97



362



131



37



WCMF



200



449



51



II



286



644



I75



27



361

-



127



WF



200

-



-



36

-



WCF



27 I



456



I I2



2s



286



338



161



54



WCMF



219



485



63



12



203



703



I03



IS



338

-



161



WF



278

-



-



48

-



WSF



216



47 I



I17



26



215



474



I08



?I



WSSF



20s



454



77



17



202



474



98



19



2-year



220



657



1 I9



18



-



-



-



-



-



213



451



I02



23



226



425



I37



3s



-



-



3-year



-



-



-



&year



228



463



63



13



227



39 1



I29



34



193



210



85



43



"Values are means across years. PASW = total soil-water profile (average depth of 1.5 m) minus soil-water profile at 15-bar water content. SWS (for a given fallow)

= soil-water profile (average depth of 1.5 m) at the end of fallow minus soil water at the beginning of fallow. PSE (for a given fallow) = (soil-water storage divided

by precipitation during that fallow period) X 100.

%orghum-l denotes sorghum in WSF and the first sorghum following wheat in WSSF.

'Sorghum-2 denotes the second sorghum following wheat in WSSF.



H. J. FAEUHANI ETAL.



2 10



I +--



Fallow



-+1



102

n



B

B 76

w



WCF



I



I



.

.



*&



3



.a



a



*ij



t



51



PI



Jan.



DW.

Year 1



Jan.

Year 2



Dec.

Year 3



Year 4



Figure 1 A time-scaled representation of the winter wheat-fallow (WF) and winter wheat-cornfallow (WCF) systems marking the beginning and ending of all crop and noncrop periods. Average

(1948-1995) monthly precipitation amounts are also shown for the Stratton experimental location.

(Numbers above bars represent percentage of yearly precipitation occurring in that month.)



wheat maturity in July to mid-September), (2) overwinter period (from fall to early May), and (3) late period (from spring to wheat planting in mid-September).

Note that the various crop and noncrop phases in the WCF system fit nicely within these periods. The noncrop period preceding corn is represented by the sum of

the early and overwinter periods, and the fallow after corn harvest corresponds to

the sum of the overwinter and late periods in WF.

The partitioning of fallow into these three periods was not arbitrary. Each phase

bears a distinct identity in regard to soil-water status, climate, precipitation, and

duration conditions (Black and Bauer, 1988),except that the climate is similar during the early and late fallow periods. The early period represents the highest

residue level in the WF cycle (and even higher in the WCF system due to remaining corn residue), the driest soil profile in the cycle, a short duration of about 3

months, and an average (1988-1995) precipitation of about 200 mm. In this period, the high residue levels coupled with dry soil profiles are ideal for enhanced in-



DRYLAND CROPPING INTENSIFICATION



211



filtration, even though evaporation potential is high. The overwinter period has the

lowest potential evaporation rates, low to medium soil-water profiles, and a long

duration of about 6 months with a mean precipitation of 186 mm-conditions favorable for potentially high storage of precipitation. The late fallow period, on the

other hand, represents the lowest residue levels in the cycle, the highest potential

evaporation rates, medium to wet soil-water profiles, and a duration of about 4

months with a mean precipitation of 261 mm. These conditions favor evaporation

and runoff.

These qualitative descriptions of the three periods of fallow in WF are represented quantitatively in Table V. This table was constructed by using the

1988-1995 data from WF and WCF (WSF at Walsh) systems in our case study.

Soil-water storage values were first calculated for the early, overwinter, and late

periods. For each fallow period in our systems, mean rates of evaporation (mm

day- I ) were determine as the ratio of fallow precipitation minus the storage to fallow duration in days.

The three periods of fallow are distinctly different (Table V). Ranked in order

of fallow efficiency, overwinter period was the most efficient (61 %), having the

lowest mean rate of evaporation per day (0.56 mm day-') and the greatest amount

of storage ( 1 1 1 mm) even though precipitation was at its lowest (at Sterling and

Stratton) during this period. The early period ranked second in terms of storage

(22 mm), efficiency ( 12%),and evaporation rate (1.86 mm day-'). The late (or the

summer of fallow) period was by far the most inefficient (-4% storage), even

though the greatest amount of precipitation (261 mm) is received during this time.

It had evaporation rates of about 2.2 mm day-'.

To simplify discussion of results, we assigned colors (zones) to each fallow period based on the intensity-of-evaporation rates during the period-the orange

zone refers to the early period, the blue zone to the overwinter period, and the red

zone to the late period. On average (see Table V), 11 1 mm (or 89%) of the total

125-mm fallow storage occurred during the blue zone. During the red zone, no water was conserved in this no-till fallow. As shown in Fig. 1, the red zone fallow

(i.e., the primary zone of inefficiency in the WF system) is precisely the period that

is replaced by corn or sorghum in the more intensified 3-year systems. It appears

that if no plants are present to use the soil-water reservoir during the red zone fallow, the atmosphere will consume it through evaporation. The red zone or the summer of fallow can be eliminated only by abandoning winter wheat. That solution

is unrealistic, since winter wheat is the comer stone of dryland agriculture in the

Great Plains. Thus, the only plausible and practical solution to the unavoidable red

zone fallow is to reduce its frequency of occurrence by intensification or summer

cropping. Inclusion of one summer crop in the WF system reduces the frequency

of occurrence of the red zone fallow by 33%-from one in every 2 years to one in

every 3 years.



Table V

Mean Precipitation (P),

Soil-Water Storage (SWS),Precipitation Storage Efficiency (PSE),Duration (D),and Daily Evaporation Rate (Er)

for Three Specific Periods of the 16Month Fallow in the Wheat-Fallow System as Affected by Location (Climate) ’hahnenW

Entire 14-month fallow in wheat-fallow system

Early period (orange zone)



Location



D

(days)



P

(mm)



SWS

(mm)



PSE



90



17



64



(%)



Sterling

Stratton

Walsh



87

111



197

189

232



- 14



9

34

-6



Mean



96



204



22



12



“Values are means for 1988-1995.



Overwinter period (blue zone)



Late period (red zone)



Er

(mmday-I)



D

(days)



P

(mm)



SWS

(mm)



PSE

(%)



Er

(nunday-’)



D

(days)



P

(mm)



SWS



PSE



(mm)



(%)



Er

(mmday-’)



1.92

1.44

2.22



180

178

252



169

146

243



114

97

123



66

66

51



0.63

0.55

0.49



129

136

102



251

310

229



- 32

14

-6



13

5

-2



2.17

2.16

2.30



1.86



203



188



111



61



0.56



122



261



-8



-4



2.21



-



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

II. Summer Fallow: A Second Look

Tải bản đầy đủ ngay(0 tr)

×