A water balance model for rain-grown, lowland rice in northern Australia

Agricultural Meteorology Elsevier Publishing Company, Amsterdam-Printed in The Netherlands

A WATER BALANCE MODEL F O R RAIN-GROWN, LOWLAND RICE IN NORTHERN AUSTRALIA A. L. CHAPMAN and W. R. KININMONTH*

Coastal Plains Research Station, Division o f Land Research, C.S.LR.O., Darwin, N.T. (Australia} Bureau o f Meteorology, Darwin, N. T. (Australia) (Received April 28, 1971) (Resubmitted July 8, 1971) ABSTRACT Chapman, A. L. and Kininmonth, W. R., 1972. A water balance model for rainlgrown, lowland rice in northern Australia. Agric. Meteorol., 10: 6 5 - 8 2 . A simple water balance model was developed for rain-grown, lowland rice on the heavy clay soils of the sub-coastal plain of the Adelaide River in northern Australia. The rice farming system is based on short duration (100 days) photoperiod-insensitive varieties. The seed bed is prepared in the flooded state with a tractor-driven rotary tiller. Aircraft are used to broadcast seed, fertilizer, and pesticides into the flooded fields. The crop is sown during mid-January to early February when rainfall and tillage criteria are satisfied. The modal is based mainly on field measurements of evapotranspiration. Estimated dates of soil saturation and depths of ponded water at Coastal Plains Research Station agreed remarkably well with observed values in five wet seasons having widely different rainfall patterns. The model was used to estimate the success frequency of rain-grown, lowland rice at Darwin, Koolpinyah, and Humpty Doe with 93, 48, and 24 years o f rainfall records respectively. A computer program was written to obtain daily estimates of soil water storage and depth of ponded water for each wet season from September 1 to May 31. In the program provision was made for maximum field pondage to be successively 6, 8, 10, and 12 inches. Rainfall was regarded as adequate providing there were (a) at least 14 days pondage 73 inches between December 31 and February 18 for wet tillage before sowing, (b) at least 80 days between sowing date and the last date at which ponded water was present on the field, (c) not more than 10 consecutive zero pondage days between 50 and 80 days after sowing, corresponding to the stage of panicle initiation through flowering. A 23-year comparison showed that on the average, the water availability was similar at the three stations, although differences sometimes occurred within individual seasons. The results indicate that the expectation of crop failure due to inadequate rainfall at Darwin is about 1 year in 30. In addition there is an expectation of major yield reduction of about 1 year in 10. The seasons of crop failure and low yield tended to occur in groups after relatively long runs of seasons of adequate rainfall. Designing for a maximum field pondage greater than 10 inches did not reduce the estimated number of crop failures at any station. The results are discussed in relation to practical farming operations. INTRODUCTION

Field studies at Coastal Plains Research Station (12°33'S, 13 l°21'E) near Darwin, N.T., during the past 5 years have demonstrated the agronomic feasibility of a system of rice * Present address: Commonwealth Meteorology Research Centre, Melbourne, Vic. (Australia).

66

A.L. CHAPMAN AND W. R. KININMONTH

production under natural rainfall alone, based on short duration ( 9 0 - 1 0 0 days) photoperiod-insensitive varieties. The seed bed is prepared in the flooded state with a tractor-driven rotary tiller. Aircraft are used to broadcast pre-germinated seed, fertilizer, and pesticides into the flooded fields. The crop is sown in mid-January to early February. The soils are fine textured estuarine clays (Christian and Stewart, 1953; Hooper, 1969) comprising the sub-coastal plain of the Adelaide River (Fig. 1).

Fig. 1. Darwin area, northern Australia, showing location of the Adelaide River and the rainfall stations referred to in the text. The climate of the Darwin region is monsoonal with a 3 - 5 month wet season of 5 0 - 6 0 inches and a 7 - 9 month dry season (Slatyer, 1960; Southern, 1966). About 90% of the rain occurs during November-March. Rain rarely falls between May and September. The reliability o f the total annual rainfall is high, but the distribution within seasons is extremely variable (Fig.2). In an unpublished report in 1954, R. O. Slatyer analysed the Darwin rainfall records in relation to rice growing. Using data then available he made various assumptions regarding the end of the dry season soil moisture deficit and evapotranspiration regimes. Other assumptions were a maximum field storage o f 10 inches of standing water and aerial sowing as soon as 2 inches of ponded water was present. He estimated that the growing season was at least 150 days in four-fifths of the years examined. In 10 out of 56 years he found that all standing water was removed for short periods but this always occurred during the early growth stages, and since the soil remained saturated it was assumed that no damage would result.

WATER BALANCE MODEL FOR RAIN-GROWN,LOWLANDRICE

67

Slatyer (1968) has discussed the use of water balance relationships in agroclirnatology. The concepts have been used successfully by Fitzpatrick et al. (1967) to estimate the incidence and duration of periods of plant growth in central Australia, and by Nix and Fitzpatrick (1969) to predict wheat and grain sorghum potential in part of central Queensland. This paper describes a water balance model for lowland rice on the Adelaide River sub-coastal plain and its use to determine the expectation of crop failure due to inadequate rainfall.

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Fig.2. Rainfall variability at Darwin for the period 1869-1967 (92 complete years) expressed as the 20 (dotted curve), 50 (dashed curve), and 80 (solid curve) percentiles for each 10-day period throughout the year. Variability is similar at Koolpinyah and Humpty Doo.

DATA AND ASSUMPTIONSUSED IN THE RICE WATER BALANCE MODEL Soil moisture deficit at the end o f the dry season By September following a wet season rice crop, the estuarine clays (80-85% < 2/~) of the sub-coastal plain have cracked into large polygonal blocks. The cracks extend into moist soil to 12 inches and deeper, but their greatest width is in the 0 - 8 inch zone. At this time cracks occupy about one-quarter of the field surface area. The soil is saturated, or nearly so, at a depth of about 16 inches. In the model it is assumed that 6 inches of water is required to saturate the soil profile at the end of the dry season. This assumption is based on: (1) A crack volume per acre equivalent to about 6 inches of water estimated from measurements at Coastal Plains Research Station in late September 1967. (2) Profile recharge data for a cracking clay under similar climatic conditions at Kimberley Research Station, W.A. (Stern, 1965). (3) Observed soil saturation and ponding of water to an average depth of 1 inch at Coastal Plains Research Station in mid-October 1968 following heavy rain totalling 6 inches over 5 days.

68

A.L. CHAPMAN AND W. R. KININMONTH

(4) Observed soil saturation and the beginning o f ponding at Coastal Plains Research Station in late October 1969 following 6 inches of rain over 7 days. The total soil water storage is partitioned into: the top 1 inch o f soil (store A), the second 1 inch o f soil (store B), and the remainder o f the soil water storage (store C) with assumed capacities o f 0.30, 0.30, and 5.40 inches, respectively. The values for stores A and B are based on experimental data for the soil in question. This procedure was adopted to accommodate the differential evapotranspiration rates observed in the upper soil layers (Fig.3). By so doing it was thought that the daily estimates of soil water storage would be more realistic. 2.0 i~

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Soil water storage recharge Soil water storage on September 1 is assumed to be zero. Rain is added first to store A, then to store B, and finally to store C until the soil is saturated and ponding occurs. The pattern of profile wetting is much more complex than this simple model which assumes horizontal boundaries between stores. Initially the surface area of store A is much greater than the surface area of the field because o f the extensive cracking of the soil. Light showers of rain wet the sides o f the soil blocks mainly by direct penetration into the cracks. During heavy falls of rain there is run-off into the cracks and on these occasions some water probably enters store C directly. The wetting and drying cycles shatter the walls o f the soil blocks. Wall shatter, together with erosion of the block edges, eventually produces a soil plug in the cracks. If the pre-monsoon showers are predominantly light the cracks below the soil plug may remain open for a long period. Loss o f water by surface r u n - o f f It is assumed that there are levees bounding the field and that the outlets remain closed from the commencement o f the first showers of the wet season until adjustment of water

WATER BALANCE MODEL FOR RAIN-GROWN, LOWLAND RICE

69

depth is necessary for wet cultivation and sowing. Water depth is maintained ~ 4 inches and -<5 inches during the first and second fortnights respectively, following sowing. Thereafter, water is allowed to accumulate to maximum pondage depth of say, 10 inches any surplus being lost by surface drainage.

Loss of water by deep drainage The permeabilities o f the Adelaide River sub-coastal plain clays are extremely low (data from an unpublished report by A. Hooper, 1966). There is no evidence o f significant downward movement o f water through the profile and in the model deep drainage is assumed to be zero.

Loss o/water by evapotranspiration In the model all evapotranspiration is assumed to take place during the daylight hours and all rainfall to occur at night. Table I shows the predominantly late afternoon and nocturnal occurrence of the rainfall especially during the pre- and post-monsoon periods o f the wet season. The error TABLE I The number of rain showers commencing between 16h00 and 06h00 and the rain recorded during this period expressed as a percentage of the total number of showers and total rain recorded for each stage of the wet season at Coastal Plains Research Station Season

Period

1967-68

Pre-monsoon (Sept. 1-Jan. 17) Monsoon (Jan. 18-Mar. 11) Post-monsoon (Mar. 12-Apr. 30)

1968-69

Pre-monsoon (Sept. I-Jan. 3) Monsoon (Jan. 4-Mar. 17) Post-monsoon (Mar. 18-Apr. 30)

Rain showers with time of onset between 16h00 and 06h00 (%) 70.3

Rain recorded between 16h00 and 06h00 (%) 66.4

45.8

42.7

80.0

79.8

72.2

69.5

62.2

65.0

89.5

98.9

introduced by assuming all rain to occur at night is greatest during the actual monsoon period, but here also, about 50% of the rain falls between 16h00 and 06h00. An examination of 12 years of rainfall at Darwin showed that 68% o f the rain fell between 16h00 and 06h00. At Coastal Plains Research Station the corresponding 2-year average was 70%.

70

A.L. CHAPMAN AND W. R. KININMONTH

Nocturnal evaporation (18h30-06h30) from a Class A pan accounted for about one-quarter of the daily total with values ranging from 8-36%. Average night/day ratios were similar for the three periods of the wet season (Coastal Plains Research Station unpublished data, 1969). No values are available for nocturnal evapotranspiration from a rice crop at Coastal Plains Research Station. However, some lysimeter data obtained at Katherine, N.T. (March-April 1967) with Stylosanthes humilis show night/day evapotranspiration ratios generally less than 0.3 (D. Watson, personal communication, 1969).

Evaporation from bare soil Following the predominantly late afternoon and nocturnal rain showers of the pre-ponding phase the soil surface is dry and cracked by local noon. Thus the observed bare soil evaporation from store A during the first 24 h after rain is considerably less than that from a free water surface. Data collected at Coastal Plains Research Station during one drying cycle in November 1967 (Fig.3) indicated that water evaporated from store A at the rate of about 0.22 Class A pan evaporation for the 1st, 2nd, and 3rd days after rain. The evaporation rate then declined rapidly to about 0.04 Class A. Evaporation from store B was negligibly small until store A was reduced to about one-fifth. Water loss then increased to about 0.11 Class A until store B was reduced to about one-third, after which evaporation fell to about 0.04 Class A. During the 1967-68 season changes in volumetric water content indicated a loss equivalent to one-half inch from store C over a 6-week period during the pre-ponding phase. The total amount of water contained in each store expressed in terms of the equivalent depth of surface water was obtained by multiplying the thickness of the soil layer by the volumetric water content. Using Fig.3 as a basis the bare soil evaporation rates (subscripted for store) assumed in the model are as follows: EA = 0.08 inch/day for 3 days following a "rain day" or until store A holds 0.06 inch and then 0.02 inch/day until store A = 0. A "rain day" is defined for this purpose as a day on which 0.08 inch or more of rain is recorded. EB -- 0 for 3 days following rain or until store A holds 0.06 inch, then 0.05 inch/day for the next 4 days or until store B holds 0.10 inch and then 0.02 inch/day until store B = 0. EC = 0 generally, but when store A = 0 and store B holds 0.10 inch,Ec = 0.01 inch/day.

Bvaporation from a flooded field Daily records of water levels in flooded fields at Coastal Plains Research Station made over 3 - 4 weekly periods in the 1966-67 and 1967-1968 wet seasons, indicated that the evaporation from a standard Australian sunken pan located in a grassed site nearby was a good approximation of the water loss from a flooded, uncropped field. The only longterm meteorological records available for the Adelaide River sub-coastal plain are daily rainfall data for two stations, Koolpinyah and Humpty Doo, situated about 12 and 27 miles

WATER BALANCE MODEL FOR RAIN-GROWN,LOWLANDRICE

71

respectively from the north coast. For Coastal Plains Research Station, short-term records exist for other daily weather data, including radiation and evaporation. Fitzpatrick (1963) derived an equation for estimating pan evaporation from mean maximum temperature and vapour pressure. The method is applicable over much of northern Australia. However, for the two stations concerned here, the required data are not available. An attempt was made to estimate daily evaporation from daily rainfall for years and stations without pan evaporation data. No relationship between rainfall and evaporation was found for periods shorter than one month, probably because most of the rain is late afternoon or nocturnal and the water vapour content of the lower atmosphere is relatively stable throughout the wet season despite large changes at high altitudes. No improvement in the estimate of evaporation was obtained by taking wet ~nd dry days into account separately as compared with using the mean daffy evaporation for each month. The mean daily evaporation values used in the model are based on the Australian sunken tank data for Coastal Hains Research Station, 1960-68. Class A pan equivalents are given below (the Australian sunken tank/Class A ratio was taken as 0.75, an average ratio found during 1968-69 wet season and over a 20-month period; values ranged from 0.64 in February to 0.85 in July). September October November December January February March April May

-

0.41 0.44 0.35 0.31 0.25 0.24 0.27 0.29 0.33

inch; inch; inch; inch; inch; inch; inch; inch; inch.

When pondage exists, evaporation from the flooded field is assumed to occur at the above rates until 2 weeks after sowing.

Evapotranspiration from a flooded field with rice crop At Los Bafios, Philippines, evapotranspiration from transplanted rice ranged from 0.07 inch to 0.31 inch/day during the wet season with an average of 0.20 inch/day. Maximum evapotranspiration occurred at the heading stage (Anonymous, 1963). Estimates of evapotranspiration rates from water-sown rice at Coastal Plains Research Station were obtained by monitoring field water levels throughout 4 wet seasons. Values ranged from 0.13 to 0.31 inch/day'with an average of 0.24 inch/day. Maximum evapotranspiration occurred during the panicle initiation to heading stage. The field/Class A pan ratios for the 1967-68 and 1968-69 seasons were 0.82 and 0.91 with an average of 0.86. The corresponding value for 1966-67 computed later, was 0.93.

72

A.L. CHAPMANAND W. R. KININMONTH

In the model an average evapotranspiration rate of 0.86 Class A is assumed from 2 weeks after sowing until the field is drained in preparation for harvest. The use of a single evapotranspiration factor throughout the crop growth period is open to criticism because it may lead to an underestimate of the lengths of dry spells during panicle initiation through heading.

Evapotranspiration from an unflooded field with rice crop Information is lacking at Coastal Plains Research Station regarding the rates of soil water extraction that can be expected after pondage is reduced to zero during active growth or crop maturation. A stepped evapotranspiration function was suggested (R. O. Slatyer, personal communication, 1967) of the general type used by Slatyer (1960) to estimate the crop growing season at Katherine, N.T. If pondage is reduced to zero after sowing, evapotranspiration is assumed as follows: 0.75 0.75 0.56 0.38 0.19

Class A from 6.00-5.70 Class A from 5.70-5.40 Class A from 5.40-4.00 Class A from 4.00-2.00 Class A from 2.00-0.00

inch water storage (store A) inch water storage (store B) inch water storage) inch water storage~ (store C) inch water storage)

These rates of soil water extraction are also assumed to apply following draining of the field in preparation for harvest. Data collected in 1968-69 indicated that the soil profile was still close to saturation 7 - 1 0 days after draining off surplus water 2 weeks before harvest. This is probably due partly to the fact that some water remains trapped in the surface root mat of the rice plants and in shallow depressions after flow from the field ceases. Thus pondage is effectively greater than zero at this stage. The data also showed that water was removed from all three stores simultaneously. Average extraction rates were about 0.02 Class A for stores A and B together and of the same order for store C. Confirmation of these results is still required. Nevertheless, if the true evapotranspiration rates are in fact lower than the assumed values, the errors introduced will not be critical. The model will, if anything, tend to overestimate the rate of soil water extraction when pondage is zero. METHODS The model was tested against dates of soil saturation and depths of ponded water observed in the field at Coastal Hains Research Station during the 1966-67, 1967-68, 1968-69, and 1969-70 wet seasons. The average depth of ponded water at any time was determined from daily records of water levels in relation to a datum at the field outlet. This datum, established in 1966-67, was based on a grid of 150 measurements of water depth per 6-acre field made with a ruler after sufficient rain had ponded on the field to cover it completely.

73

WATER BALANCE MODEL F O R RAIN-GROWN, LOWLAND RICE

Despite the many assumptions used in the model there was remarkably good agreement between the estimated and observed soil saturation dates and water depths for widely different rainfall patterns (Fig.4, Table II). For example, in the 1967-68 season soil saturation and pending occurred on January 19, two days after the onset of the monsoon. The first thunderstorm occurred on October 7. Between that date and January 17, 41 falls of rain were recorded, only one of which (on January 2) was more than 1 inch. In

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WATER BALANCEMODELFOR RAIN-GROWN,LOWLANDRICE

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1968-69, ponding first occurred on October 15 after only six falls of rain, of which three were greater than 1 inch and two were almost 3 inches. The discrepancy between estimated and observed soil saturation dates and ponding in October 1968 may have been due to the unusually heavy (10 inches) rain in May. Under these circumstances the soil moisture deficit at the end of the dry season may have been less than the 6 inches assumed in the model. Also, because of the intensity (6 inches in 5 days) of the October rain, soil swelling and reduced infiltration may have resulted in ponding before the soil profile was fully saturated. The latter interpretation is supported by the observation in October 1969 that 6 inches of rain over 7 days resulted in soil saturation and the beginning of ponding. On this occasion the discrepancy between the estimated and observed values was much smaller. In simulating the rice cropping system, the following assumptions are made: (1) The soil is left undisturbed during the dry season; residues of the previous rice crop are burned. (2) Weed control is achieved initially by normal cultivation as required following the early thunderstorm rains. (3) Seed bed preparation and weed control before sowing is done by rotary tillage in the flooded state after sufficient rain water has ponded on the field. (4) The growing period of the rice variety is 100 days from sowing to rnaturity. (5) A minimum of 80 days pondage is required before the field is drained in preparation for harvest. The model was then applied to the rainfall data for Darwin, Koolpinyah, and Humpty Doo with 93, 48, and 24 years of records, respectively. A computer program was written to obtain daily estimates of soil water storage (partitioned into A, B, and C as above) and depth of ponded water for each wet season, September 1 to May 31. In the program provision was made for maximum field pondage to be successively 6, 8, 10, and 12 inches. There was also a "no drainage at 80 days after sowing" mode. In this case an evapotranspiration rate of 0.86 Class A was assumed until pondage was reduced to zero. An estimate was thus obtained of the maximum possible duration of ponding as well as the effect of maximum field pondage depth on ponding duration. Other information obtained from the computer program included the following: (1) Soil saturation date, i.e. the first date at which the end of dry season soil water deficit is zero. No more water can be absorbed by the soil and further rain results in ponding on the field. (2) Wet cultivation date, i.e. the first date at which pondage is ) 3 inches. (3) Time available for wet cultivation, i.e. the number of days that pondage is ) 3 inches during the 50-day period, December 31 to February 18. (4) Sowing date A, i.e. January 15 providing pondage is ) 3 inches on that date and there has been at least 14 days pondage ) 3 inches since December 31. Otherwise sowing is delayed until these requirements have been met. (5) Sowing date B, i.e. January 15 providing pondage on that date is ) 3 inches or the first day thereafter that pondage is ) 3 inches.

76

A.L. CHAPMAN AND W. R. K1NINMONTH

(6) Last pending date, i.e. the last date at which ponded water is present on the field; the soil is still saturated. (7) Identity and number o f zero-pondage days between sowing date A and last pending date. Rainfall was regarded as adequate for lowland rice if the following criteria were satisfied: (1) At least 14 days pondage/>3 inches between December 31 and February 18 for wet cultivation before sowing. (2) A period o f at least 80 days between sowing date A and last pending date. (3) Not more than 10 consecutive days with zero-pondage between 50 and 80 days after sowing, i.e. panicle initiation through flowering. RESULTS Soil saturation date Estimated soil saturation dates ranged from October 23 to January 9 at Darwin, November 1 to January 3 at Koolpinyah, and October 23 to January 4 at H u m p t y Doe (Fig.5, a). In 88% o f the years at Darwin and H u m p t y Doe and 96% o f the years at Koolpinyah the soil was saturated by the end o f December. I00

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WATER BALANCEMODELFOR RAIN-GROWN,LOWLANDRICE

77

The estimated "permanent" soil saturation dates (Fig.5, b) ranged from November 10 to February 22 at Darwin, November 25 to February 7 at Koolpinyah, and November 14 to January 25 at Humpty Doo. In 91% of the years at Darwin and 96% of the years at Koolpinyah and Humpty Doo "permanent" soil saturation occurred by January 15. Wet cultivation date The estimated wet cultivation date was January 15 or earlier in 75% of the years at Darwin, 81% at Koolpinyah, and 58% of the years at Humpty Doo (Fig.5, c). In 95% of the years at Darwin, 98% at Koolpinyah, and in all years at Humpty Doo, 3 inches of ponded water was present by January 31. Wet cultivation date was February 14 or earlier in all years at Koolpinyah and Humpty Doo and in 92 out of 93 years at Darwin. In the one season at Darwin (1905-06) estimated pondage did not reach 3 inches until February 26. Time available for wet cultivation It was estimated that at least 10 days during the 50-day period December 31 to February 18, were available for wet tillage in all years at Humpty Doo, 98% at Koolpinyah, and 97% at Darwin (Fig.6A). In three-quarters of the years at Humpty Doo, 86% at Darwin, and 94% at Koolpinyah the time available for wet cultivation was estimated to be at least 20 days. The estimated number of pondage days available for wet tillage was less than the 14-day minimum specified for sowing date A, in 7% of years at Darwin, 4% at Koolpinyah, and 8% at Humpty Doo. Estimated deficits ranged from 1 day in 1948-49 at Humpty Doo to 14 days in 1905-06 at Darwin. Sowing date Estimated sowing date A (defined in legend to Fig.6B) fell in the period January 15-31 in 73% of the years at Darwin, 75% at Koolpinyah, and 54% at Humpty Doo (Fig.6B, 1). Estimated sowing date B fell during January 15-19 in 84% of the years at Darwin, 85% at Koolpinyah, and 67% at Humpty Doo. Sowing date B was estimated to be no later than the end of January in all of the years at Humpty Doo and in 96 and 95% of the years respectively at Koolpinyah and Darwin. Duration o f ponding The estimated ponding duration (Fig.6C), i.e. the number of days between sowing date A and last ponding date was ~80 days in 90% of the years at Darwin, 96% at Koolpinyah, and 83% at Humpty Doo. The estimated time interval between sowing date B and last ponding date was ~80 days in 96% of the years at Koolpinyah and Humpty Doo and 97% of the years at Darwin.

78

A.L. CHAPMAN AND W. R. KININMONTH

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rb ' ~io' d o ' 1be' rio' 1 ~ ' ;~o' lka Days

Fig.6. Sowing dates, time available for wet cultivation, and pending duration for Darwin (93 years). Similar results were obtained for Koolpinyah and Humpty Doe. A. Time available for wet cultivation, i.e. number of days that pondage 93 inches between December 31 and February 18. B. Sowing date A; 1 = January 15 providing pondage on that date is ~3 inches and there have been at least 14 days with pondage ;~3 inches since December 31 or the first date after January 15 when these requirements have been met. Sowing date B; 2 = January 15 providing pondage on that date ~3 inches or the f'trst date thereafter that pondage 93 inches. C. Pending duration, i.e. number of days between estimated sowing date A and estimated last pending date. Maximum field pondage is 10 inches.

The effect of maximum field pondage depth (Pmax)on pending duration W h e n Pmax was increased f r o m 6 t o 12 i n c h e s t h e r e was n o effect o n soil s a t u r a t i o n d a t e , w e t c u l t i v a t i o n d a t e , s o w i n g d a t e , or t h e n u m b e r o f days p o n d a g e ) 3 inches b e t w e e n D e c e m b e r 3 ! a n d F e b r u a r y 18 at a n y o f t h e t h r e e stations. P e n d i n g d u r a t i o n was increased b y a n average o f 16 days at e a c h s t a t i o n , a n d occasionally as m u c h as 4 t o 6 weeks. H o w e v e r , i n 15% o f years at D a r w i n , 10% at K o o l p i n y a h , a n d 12% at H u m p t y D o e t h e r e was little or n o effect o n p e n d i n g d u r a t i o n . T h e n u m b e r o f years in w h i c h p e n d i n g d u r a t i o n was ) 8 0 days was d e c r e a s e d b y 2% a n d 4 % respectively at D a r w i n a n d K o o l p i n y a h at Pmax = 8 i n c h e s a n d a f u r t h e r 8% at b o t h s t a t i o n s at Pmax = 6 inches. T h e c o r r e s p o n d i n g decreases were 12 a n d 4% at H u m p t y D o e . A b o v e Pmax = 10 i n c h e s t h e r e was a n increase o f 2% at D a r w i n b u t n o e f f e c t at Koolpinyah and Humpty Doe.

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79

Incidence o f mid-season dry spells In 14% of the years at Darwin, 12% at Koolpinyah, and 21% at Humpty Doe it was estimated that pondage was reduced to zero during dry spells in the middle of the wet season. The frequency and duration of zero-pondage periods between sowing date A and last ponding date were reduced in a few years at all stations by setting Pmax = 8 inches. Above this value Pmax had little or no effect on the incidence of zero-pondage days. In only one year at Darwin was there any advantage in setting Pmax at 10 inches. Generally the periods without ponded water were less than one week at all stations. However, there were 5 years at Darwin, 2 years at Koolpinyah, and 1 year at Humpty Doe in which these periods were estimated to be of 1 0 - 2 0 days duration. At Darwin, pondage was estimated to be zero for 13 consecutive days during panicle emergence and flowering in 1 9 1 0 - 1 1 , 2 0 days beginning 3 weeks after sowing in 1930-31, and for a total of 18 days out of a 21-day period starting just before panicle initiation in 1958-59. At Koolpinyah in 1930-31, pondage was zero for 11 consecutive days just before panicle initiation. In 1958-59, there were 11 days of zero pondage just after panicle initiation at Koolpinyah and 18 days ending just before panicle initiation at Humpty Doe.

Comparison of wet season at Darwin, Koolpinyah and Humpty Doo A comparison of 23 wet seasons (1942-43 to 1964-65) at Darwin, Koolpinyah, and Humpty Doe (Table III) showed that the average number of days pondage >/3 inches TABLE III Average values of several estimated parameters at Darwin, Koolpinyah, and Humpty Doe for the 23-year period 1942-43 to 1964-65 Estimated parameter

Darwin

Koolpinyah

Humpty Doe

First soil saturation date "Permanent" soil saturation date Wet cultivation date Time available for wet cultivation (days) Sowing date A Sowing date B Last pending date Pending duration for sowing date A (days)

Dec.3 Dec.21 Dec.29 35 Jan.24 Jan.17 May 11 107

Dec.5 Dec.22 Dec.30 38 Jan.24 Jan.19 May 10 106

Dec.9 Dec.23 Jan.6 32 Jan.29 Jan.19 May 9 100

between December 31 and February 18 at Humpty Doe was 6 less than at Koolpinyah and differed significantly from the latter. Otherwise, differences in average soil saturation, wet cultivation, and last pending dates and in pending duration among the three stations were not significant.

80

A. L, CHAPMANAND W. R, KININMONTH

DISCUSSION In estimating the water available for lowland rice, the model assumes that sowing is begun only after all tillage is completed. The main purpose of wet tillage is the control of weed growth during the early stages of crop establishment. An important secondary objective is to create a seed bed having the surface roughness and other physical characteristics essential for the establishment of water-sown rice. The optimum time interval between wet tillage and sowing is determined by these requirements. If wet tillage is done too early, weeds grow again before sowing, soil surface roughness is reduced by wave action and the mechanical resistance of the soil may become too great (especially if pondage is reduced to zero again) so that further tillage is necessary. For best results wet tillage should precede sowing by 2 - 3 days. This means that the farmer cannot take advantage of the fact that in about one-third of the years there is sufficient ponded water for wet tillage by December 25. In firm soil, shallow water and light weed growth, the best tillage rate achieved so far is about 2½ acres/h using a 12-ft. rotary tiller mounted on a 65 h.p. wheeled tractor fitted with steel halftracks. The effective wet season is short and no more than one pass over the field can be afforded, In practice, sowing would commence before tillage of the whole area is completed in order to take advantage of favourable weather, to achieve the staggering of harvest dates theoretically possible with a photoperiod-insensitive variety, and to comply with seed bed requirements. The fact that doubling Prnax from 6 to 12 inches increased the number of years in which ponding duration was i>80 days by only 12% at Darwin can be explained by the low probability of receiving heavy falls of rain late in the season. Also the crop is sown at the time when rainfall reliability is highest and pondage is often lost because of the need to maintain relatively shallow water during the first month after sowing. Consequently, designing for a Pmax greater than 10 inches had no effect on the estimated number of crop failures at any station. In predicting the stage of crop growth at the time of onset of a zero-pondage period, it is assumed that the whole area is sown on the same day. According to Matsushima (1961) the greatest reduction in yield can be expected when the water stress occurs during the period 20 days before to 5 days after 50% panicle emergence. When pondage was reduced to zero at panicle initiation at Coastal Plains Research Station in 1969-70, 20 days elapsed before any plants showed obvious signs of water stress during the daylight hours. Plant population was low and la,~ inches of rain were recorded during this period. The assumed upper limit of 10 consecutive days with zero pondage between panicle initiation and the end of flowering, probably overestimates the effects of mid-season dry spells on grain yield. The general similarity of the wet season at the three stations is undoubtedly due to their proximity to each other and to the low relief of the whole area. Few parts of the terrain rise to more than 100 ft. above mean sea level, Nevertheless, differences sometimes occurred within individual seasons.

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81

In the absence of long-term rainfall records at Humpty Doo, a fairly reliable estimate of crop success frequency at this station can be obtained from the Darwin data. The results indicate that the long-term expectation of crop failure due to inadequate rainfall at Darwin is about one year in 30 with an expectation of major yield reduction of about one year in 10. However, the poor seasons tended to occur in groups after relatively long runs of good seasons. For example, there was a long run of good seasons from 1869-70 to 1899-1900. The following 6-year period 1900-01 to 1905-06 included one year of total crop failure and two with low yields. Records of rice culture by the Chinese in the Darwin area between 1884 and 1910 confirm that the crop failed for the first time in 1905-06 after a long series of successes (Bauer, 1964), The 5-year period 1926-27 to 1930-31 which included one year of total crop failure and 3 years with low yields, also occurred after a long run of years with adequate rainfall. The estimate of crop success frequency obtained in this analysis is a conservative one because of the limits set in the model. Experience at Coastal Plains Research Station indicates that seed bed preparation is often possible with less than 3 inches of ponded water. This is especially true where the soil has been dry cultivated following the early thunderstorm rains. Pondage limits during the month after sowing are also more flexible in practice. If the prospects of further rain are poor, as determined from the regional weather forecast, more water may be conserved. Depending on water temperature and wind satisfactory seedling establishment can be obtained in 6 inches of standing water. The substitution of herbicides for wet tillage would make extra pondage available for crop growth. However, chemicals are still more expensive than mechanical cultivation. Also, the problems of poor root penetration (resulting in greater seedling loss through drift) and enhanced crop lodging remain to be solved. A sowing date much earlier than January 15 is not recommended for a 100-day variety because of the chance of receiving heavy rain sufficient to interfere with harvesting. In 11% of the years at Darwin, 4 inches or more of rain and probably sufficient to interfere with harvest, was recorded after April 10. From Fig.6C it is clear that a short-duration variety is essential for the success of the proposed rice farming system. Pondingduration was too short for a 110-day variety in 15% of the years at Darwin. The expectation of crop failure due to end of season drought is much greater for varieties with growth durations of 120 days or more. This type of analysis could be extended to other stations with suitable rice soils in the 4 5 - 6 0 inch rainfall zone of northern Australia wherever records of sufficient length exist. The model parameters can be varied to examine the success frequency of different farming systems. ACKNOWLEDGEMENTS The authors are indebted to Professor R. O. Slatyer, Australian National University, whose early work formed the basis of the project and whose continued interest has been so helpful.

82

A.L. CHAPMAN AND W. R. KININMONTH Thanks are due to the following staff of the Bureau of Meteorology-Miss M. McCarthy

(since resigned) and Mr. P. Fenwick for programming the computer analysis of the rainfall records, Mr. C. Pierrehumbert for the computation of the percentile data, Mr. F. A. Powell, Mr. G. U. Wilson and Mr. C, E, Hounam for helpful advice and co-operation during the project. The assistance of Mrs. G. Keig in programming, Mr. G. P. Fromm and Mr. C. M. Patrick in data collection and computation is also gratefully acknowledged. We wish to thank Dr. C. W. Rose, Mr. H. A. Nix and Mr. G. A. Stewart for their comments on the text and suggestions regarding data presentation. Mr. Kininmonth's contribution to this paper is published by permission of the Director, Commonwealth Bureau of Meteorology, Australia, REFERENCES Anonymous, 1963. Annual Report. The International Rice Research Institute, Los Bafios, Laguna, Philippines, 199 pp. Bauer, F. H., 1964. Historical geography of white settlement in part of northern Australia, 2. The Katherine-Darwin region. Aust. C.S.LR.O. Div. Land Res., Reg. Surv., Div. Rept., 64(1): 178-179. Christian, C. S. and Stewart, G. A., 1953. General report on survey of Katherine-Darwin region, 1946. Aust. C.S.I.R.O., Land Res. Ser., 1:156 pp. Fitzpatrick, E. A., 1963. Estimates of pan evaporation from mean maximum temperature and vapour pressure. J. Appl. Meteorol., 2: 780-792. Fitzpatrick, E. A., Slatyer, R. O. and Krishnan, A. I., 1967. Incidence and duration of periods of plant growth in central Australia as estimated from climatic data. Agric. Meteorol., 4: 389-404. Hooper, A. D. L., 1969. Soils of the Adelaide-Alligator area. Aust. C.S.LR.O. Land Res. Set., 25: 95-113, Matsushima, S., 1961. Some experiments on soil-water-plant relationships in the cultivation of rice. IRC WorkingParty on Rice Production and Protection, 9th Meeting, New Delhi, pp. 1-2. Nix, H. A. and Fitzpatrick, E. A., 1969. An index of crop water stress related to wheat and grain sorghum yields. Agric. Meteorol., 6: 321-337. Slatyer, R. O., 1960. Agricultural climatology of the Katherine area, N.T. Aust. C.S.LR.O., Div. Land Res., Regional Surv. Tech. Pap., 13:39 pp. Slatyer, R. O., 1968. The use of soil-water-balancerelationships in agroclimatology. UNESCONat. Resour., 7: 73-89. Southern, R. L., 1966. A review of weather disturbances controlling the distribution of rainfall in the Darwin-Katherine region, N.T. Aust. Bur. Meteorol. Working Pap., 65/3203, May 1966, 15 pp. Stern, W. R., 1965. Provisional estimates of crop evapotranspiration in the Ord River valley. Aust. CS.I.R.O., Div. LandRes., Reg. Surv. Tech. Mere., 65(6): 1-11.