The occurrence of agricultural drought at Ashburton, New Zealand

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1 New Zealand Journal of Agricultural Research ISSN: (Print) (Online) Journal homepage: The occurrence of agricultural drought at Ashburton, New Zealand D. S. Rickard To cite this article: D. S. Rickard (196) The occurrence of agricultural drought at Ashburton, New Zealand, New Zealand Journal of Agricultural Research, 3:3, , DOI: 1.18/ To link to this article: Published online: 12 Jan 212. Submit your article to this journal Article views: 187 View related articles Citing articles: 9 View citing articles Full Terms & Conditions of access and use can be found at

2 196) 431 THE OCCURRENCE OF AGRICULTURAL DROUGHT AT ASHBURTON, NEW ZEALAND By D. S. RICKARD, Winchmore Irrigation Research Station, Ashburton (Received for publication, 9 February 196) Summary Agricultural drought is defined as eisting when the soil moisture in the root zone is at, or below, the permanent wilting percentage. The condition continues until rain falls in ecess of the daily evapotranspiration. The occurrence of agricultural drought at Ashburton was calculated for 44 seasons, using Thornthwaite's method of estimating changes in soil moisture. Agricultural drought occurred in every season studied, ranging from 17 days ( ) to 6 days ( ). The average number of drought days was 59. Nearly 6% of all days of drought occurred in spells of 1 consecutive days or more. The data obtained gave no indication of any periodicity in the occurrence of drought. The seasonal production of pasture and lucerne was negatively correlated with the total number of days of drought in a season, the correlation coefficients being -.91 for lucerne, and -.92, -.58 and for pasture measured from three eperiments. INTRODUCTION Rainfall in New Zealand does not vary greatly from year to year, and is, generally, reasonably well distributed throughout the year. Some regions, however, particularly in the South Island, eperience on occasions moderate to severe dry periods. These dry periods can cause considerable loss of farming production: for eample, prolonged dry weather during the season had a near-disastrous effect on many farmers in Banks Peninsula and Marlborough. Drought, therefore, is probably the most important of the natural hazards affecting farming in New Zealand. In many places it is also the one climatic etreme which can be, at least partially, overcome-by irrigation. It is important, therefore, that the likely occurrence, distribution, and effect of drought be studied. Thornthwaite and Mather (1955) have listed four types of drought: (1) Permanent Drought: driest climate, no agriculture without irrigation; (2) Seasonal Drought: areas with well-defined rainy :md dry seasons: (3) Contingent Drought: depending on the irregularity of rainfall and occurring in sub-humid and humid climates; (4) Invisible Drought: where rain does not equal evapcltram;;iration (most regions). The value of number (4), invisible drought, is doubtful: the term 'drought' should be confined to fairly serious moisture deficits. Contingent droughts are the main concern in New Zealand and the present N.Z. J. agric. Rfs. 3: 431~441

3 432 NEW ZEALAND JOURNAL OF AGRICULTURAL RESEARCH (JUNE study was undertaken to determine the frequency, duration, and agricultural significance of such droughts at Ashburton. Dry periods and periods of low rainfall in New Zealand have been described by Kidson (1931) and drought by Bondy (195). In the latter, the generally accepted definitions of drought have been used. These are: An absolute drought is a period of at least 15 consecutive days, to none of which is credited.1 in. or more of rain. A dry spell is a period of at least 15 consecutive days, to none of which is credited.4 in. or more of rain. A partial drought is a period of at least 29 consecutive days, during which the mean daily rainfall does not eceed.1 in. per day. Rainfall has been used as the basis for determining the occurrence of drought mainly because it is easily measured and information on it is readily available. It will be realised, however, that such definitions may bear little relationship to drought as eperienced in agriculture, for we are here concerned with the growing plant and its moisture supply. Fifteen consecutive days without rain in mid winter in Canterbury would probably result in only a slight lowering of the percentage of moisture in the soil from field capacity, but the same rainless period in mid summer may bring the soil from field capacity to close to the permanent wilting percentage. At this point a fall of rain of.1 in. would technically break the 'drought' but would have no effect on the dry soil. An accurate definition of agricultural drought, therefore, should take into consideration the moisture state of the soil, and its effect on plant growth. Van Bavel and Verlinden (1956) have defined agricultural drought as "a condition in which there is insufficient soil moisture available to a crop". They further define a "drought-day" as a period of one day during which the drought condition occurs. Whether soil moisture can be said to be sufficient or insufficient for plant growth depends on a knowledge of the characteristics of the soil and the crop being considered. This definition has thus deliberately refrained from describing what is meant by "insufficient soil moisture" because of a lack of data which would enable this to be done. In the present investigation enough information was available to enable the definition to be re-phrased: "Agricultural drought eists when the soil moisture in the root zone is at, or below, the permanent wilting percentage. The condition continues until rain falls in ecess of the daily evapotranspiration." This covers only fairly severe droughts, as crop or pasture growth will probably be adversely affected before the permanent wilting percentage is reached. However, there eists some difference of opinion as to the relative availability of soil moisture over the range from field capacity to permanent wilting percentage (and this may vary between different soils) while there is general agreement that plant growth does not occur below the permanent wilting percentage.

4 196) RICKARD-AGRICULTURAL DROUGHT 433 The above definition implies a knowledge of changes in soil moisture conditions, and such records are rare. If, therefore, dependence were placed on the availability of reliable soil moisture data, the definition would have etremely restricted application. There are, however, methods available for estimating daily evapotranspiration values, and these can be used to calculate changes in the moisture level in the soil. The Thornthwaite (1948) method has been used in such drought studies by Van Bavel (1953), the Penman (1948) method by Van Bave! and Verlinden (1956) and Van Bavel and Carreker (1957) and the Dlaney Criddle method by Palmer (1958). Rickard (1957) has shown that Thornthwaite's method was reliable for local conditions, and it was therefore used. It has the advantages of simplicity and requires only daily rainfall and mean temperatures. These were available from , and the survey of drought covered the period from then to a total of 44 seasons. METHODS Changes in soil moisture levels were calculated from 1 September to 3 April, using mean daily values of evapotranspiration for each month, and daily rainfall figures. Changes in soil moisture were calculated until a deficit of 2.4 in. was obtained, corresponding to permanent wilting percentage in the top 12 in. of the Lismore stony silt loam. This soil type, which is representative of a large area of Canterbury, has an average depth of in. A root-zone depth of 12 in. is therefore reasonable for the pastures and crops grown. Once a deficit corresponding to the permanent wilting percentage was obtained, each subsequent day was a day of agricultural drought until rain fell in ecess of the daily evapotranspiration rate. The total number of days of agricultural drought for each month and each season, and the occurrence and duration of periods of 1 or more consecutive days of agricultural drought in each sea~on was obtained. RESULTS Occurrence of Agricult'ural Drought The total number of days of drought in each season vari~d from 17 ( ) to 6 ( ) with a mean of 59. The average distribution throughout the season is shown in Table 1. At the beginning of the season (1 September) the soil is at field capacity, holding just over 2 in. available water in the top 12 in. of soil. The average total evapotranspiration for September is 1.5 in. so that, even with completely rainless conditions, there is insufficient evapotranspiration to deplete the moisture held in the soil. A mean rainfall of 2.49 in. during September means that usually the accumulated loss by evapotranspiration by the end of September is not great, and October, therefore, generally commences with a reasonable store of moisture in the soil. Consequently, drought conditions never occur during September and only occasionally in October. The rate of evapotranspiration is high during December to March, and 8% of the total number of days of

5 434 NEW ZEALAND JOURNAL OF AGRICULTURAL RESEARCH (JUNE drought occur in these months. driest months. January and February are the two TABLE 1. DISTRIBUTION OF DAYS OF AGRICULTURAL DROUGHT 11NTH BY MONTH Month Total Number of Days of Agricultural Drought- 44 Seasons (Ir) of Total -_ _.._ September October November December January February March April nil _.._ _ Total Mean TABLE 2. CLASSIFICATION OF SEASONS BASED ON TOTAL NUMBER OF DAYS OF DROUGHT No. of Days of Description No. of (Ir) of Total Drought Seasons Se:lsons very dry dry moderately dry wet very wet Total I The total number of days of drought in any particular season is a measure of the degree of dryness of that season, and provides a basis for the classification of seasons. This is shown in Table 2. In 52% of all seasons, the days of drought eceeded 61 or a total of approimately 2 months. More seasons fall in the "dry" division (61-9 days) than any other. The frequency distribution of drought days in groups of 1 is given in Fig. 1: this shows that in 75% of all seasons, the number of days of drought is greater than 4, in 52% greater than 6 and in 25% greater than 8. Although the total number of drought days in a season is of importance, it is the duration of consecutive drought days which offers the greatest hazard to farming. A continuous period of 1 or more days of drought is a severe check to crop or pasture production, particularly as it occurs after a relatively rainless period. During the 44 seasons studied, there were 9 such periods, as many as five in one

6 196) RICKARD-AGRICULTURAL DROUGHT (J) z 6 o (J) «w (J) 4 lj.. o a 2 z - ~...- ~ r-- r-- - r "1 OF SEASONS DAYS OF DROUGHT> 4 52"1' >6 25"1',.8 7 w (9 ~5 z W u a:: w2... ' I! I I I I I!! Jge"",! TOTAL DROUGHT DAYS PER SEASON Fig. I.-Frequency distribution of drought days. season ( , ), and ranging up to 42 days duration (28 Nov to 8 Jan. 1917). Nearly 6% of all days of drought occurred in spells of 1 consecutive days or more. The seasonal distribution and length of such periods for the 44 seasons is given in Fig. 2. Such periods only occasionally occur in November and can occur up to the end of April and-in two cases-cven in May. It can be seen from Fig. 2 that periods of drought are frequently followed within a few days by another drought period. For eample, during the season, a drought eisted from 11 to 22 January. This was techniolly broken by 13 points of rain on January 23, and drought conditions eisted from 24 January to 5 February. Again, 61 points of rain on February 6 interrupted the drought until February 11. This last period of drought lated until 7 March. It is etremely unlikely that the two falls of rain would confer much practical advantage on a pasture or crop, and the effective total of consecutive drought days in this case was at least 56 days. As the commcncement of agricultural drought

7 436 NEW ZEALAND JOURNAL OF AGRICULTURAL RESEARCh (JUNE H~ =-----t "2,., ~.. L L_ SEPT. OCT. --, NOV. _L DEC. JAN. FEB. MAR. APR. Fig. 2.-Seasonal distribution and length of drought periods. on 11 January was preceded by 16 days with a total rainfall of.1 in. a crop or pasture would probably be suffering from a shortage of moisture before that date, and the total effect of the dry w~ather would be even more severe than indicated by the actual drought period. Possible Periodicity of Drought The data obtained were eamined by Mr N. S. MC'untier (pers. comm.) using a method suggested by Dr P. Whittle, involving the calculation of correlations of th~ graph with itself moved back a year, two years, etc. The drought data gave no real indication of periodicity. Drought and Agricultural Production Although a considerable amount has been published on the effect of soil moisture deficits on plant growth under controlled laboratory or glasshouse conditions, there is little information on the quantitative effect of drought on the field production of pastures and crops. In some cases the definition of drought has been based somewhat loosely on the effect on production of some particular crop. For eample, Barger and Thorn (1949) stated that "the term drought as employed in this study refers to a specific period of time during which the total amount of rainfall recorded at a station is deficient to the etent that, more often than not, the corn yield falls below normal for the county in which the station is situated". Hurlbut (1957) used Ladino clover as a criterion of drought in much the same way as in the present study. That is, he considered that Ladino clover could withdraw 4 in. of water

8 196) RICKARD-AGRICULTURAL DROUGHT 437 from the soil before any reduction of yield was noticed. This deficit of 4 in. marked the beginning of a period of drought, which ended when daily rainfall eceeded the water need of a healthy crop. This, in effect, is the definition used in the present study applied to the rooting characteristics of a particular crop. Parks and Knetsch (1959) in studying corn yields as affected by nitrogen topdressing and drought intensity, obtained a regression equation connecting the three. The use of a maturing crop, such as corn, complicates the picture in that drought will have a different effect at different stages of growth. The corn-growing period of 1 days was, therefore, divided into four periods, and a drought inde derived by weighting the number of drought days that occurred in each period. Their results clearly showed the limiting effect that drought can have on fertiliser response. It would be possible, from this type of data, to etend the results of a relatively few seasons' work to other seasons with different climate conditions. At Winchmore, production figures from several non-irrigated plots on field eperiments were available for a number of seasons, and it was considered that the possible relationship between production in a season and the etent of drought in that season should be investigated. Records were available from the to the seasons. Although outside the period of the original investigation, the and seasons were included in order to provide the maimum data. The production records used are comparable only within the particular field eperiment: mowing frequency, topdressing, and grazing management all have a marked effect on the measured production. Results were used from lucerne and pasture production trials; and no attempt was made to weight the importance of drought occurring at different times of a season. 1 Q y;9732-s3 r; -,91 w 8~: :: U ::'l: ~ Z 6 :: u.. I :: 4 W f-- f-- <I: 2: >- a:: 2 I \ DROUGHT DAYS PER SEASON Fig. 3.-Effect of drought days on lucerne production.

9 4SS NEW ZEALAND JOURNAL OF AGRICULTURAL RESEARCH (JUNE Lucerne. Results were available from two trials, on two different areas. The first season's production was omitted in both cases. The results are given in Fig. 3. The regression equation for deriving drymatter production (lb) from the number of drought days was y = (1) where y is the seasonal lucerne production and the total number of drought days in the season. The correlation coefficient r = Lucerne production, as measured under the conditions of the eperiment, is, therefore, strongly negatively correlated with the number of drought days in a season. The greatest number of drought days recorded was 17; this corresponds to a lucerne production of 461 lb. or 41.7% of the production epected in a season completely free from drought. Pasture. Non-irrigated production records were available for eight seasons from two consecutive Rate of Growth trials; from to Production has been graphed against days of drought in Fig. 4, and the following regression equation derivedy = , r = (2) 1 S W ::: 8 ~ '"~ y = r = - 92 z a ::: [L ::: W I- ~ L >- ::: DROUGHT DAYS PER SEASON 12 Fig. 4.-Effect of drought days on seasonal pasture production on non-irrigated area. Where y is seasonal pasture production, and is the number of days of drought in the season. Again, pasture production (as measured under the conditions of the eperiment) is strongly correlated with the number of drought days. The slope of the regression line is almost identical with that obtained for lucerne. Production in an average season of 59 drought days would be about 5% of that in a season with no agricultural drought.

10 19() RICKARD-AGRICULTURAL DROUGHT 439 Production records were also available from the non-irrigated area of another pasture production trial, for the seasons to Seasonal pasture production, graphed against the number of drought days, is shown in Fig. 5. The following regression equation was derivedy = , r = (3) a :2 la-- w a:: <J. 8 ~ z. a:: ll a:: w I-- I-- <t 2 2 a:: >- y = r= DROUGHT DAYS PER SEASON Fig. 5.-Effect of drought days on seasonal pasture production on another non-irrigated area. Although production is still negatively correlated with the number of drought days, the correlation coefficient is lower, and the regression line has a different slope, from the previous eamples. In considering the relationship between production in a season and the number of days of drought eperienced during that season, the possible effect of previous seasons-particularly if dry-should not be ignored. Two or three successive dry seasons would be epected to have a detrimental effect on pasture composition and growth. Under these circumstances, the number of days of drought in a season would have to be weighted to take into consideration the etent of drought in the previous season, or seasons. Non-irrigated pasture production figures were available from one treatment on a field eperiment in which irrigation and no-irrigation were applied in alternate seasons. Production figures were available from four non-irrigated seasons, and these were graphed against the number of drought days in Fig. 6. Any effect due to a dry previous season will be eliminated from these figures. The regression equation wasy = _.. (4)

11 44 NEW ZEALAND JOURNAL OF AGRICULTURAL RESEARCH (JUNE lh 8 y $2. r ~-'95 W (5 8 <J: '::.. I X.n 6 Z o a :: C'... 4 C!: W r- ~ 2: 2 r- :: o I,! ~ I I I 2 4 GO DROUGHT DAYS PER SEASON Fig. 5.-Effect of drought days on pasture production for non-irrigated seasons on area where irrigation and non-irrigation were applied in alternate seasons. The correlation coefficient was The slope of this line is almost identical with the lines of equations (1) and (2). It seems likely, therefore, that although previous seasons might be epected to eercise an effect, the slope of lines (1) and (2) was not, over the seasons studied, materially affected by this factor. CONCLUSIONS The method outlined of defining drought with specific reference to the moisture state of the soil has obvious advantages in agricultural work. The chief disadvantage is the lack of suitable soil moisture measurements, and if one of the methods for estimating these from meteorological data is used, it is important that the suitability and accuracy of the method chosen be established. This is at present being carried out for some selected areas in New Zealand. A good correlation eists between the total days of drought in a season and the production of lucerne and pasture as measured in a number of field eperiments. In an average season of 59 drought days production ranged from approimately 5% to 7% of that in a drought-free season. Production in a drought-free season will be lower than the production obtained under optimum irrigation. The term "drought-free season" means that the soil moisture at no time during the season reached the wilting percentage, but it could be close enough to this to affect production. With optimum irrigation, production would be higher, and the percentage reduction due to drought in an average season correspondingly greater.

12 196) RICKARD-AGRICULTURAL DROUGHT 441 ACKNOWLEDGMENTS Thanks are due to Mr P. D. Fitzgerald for calculating the regression equations, to Mr N. S. Mountier for advice and criticism, and to the Superintendent, Winchmore Irrigation Research Station, for permission to quote dry-matter production figures. REFERENCES Barger, G. L.; Thorn, H. C. S. 1949: A Method for Charactensing Drought Intensity in Iowa. Agron. J. 41: Bondy, F. 195: Droughts in New Zealand. N.Z.]. Sci. Tech. B32: 1-1. Hurlbut, L. W. 1957: Plan Ahead for Droughts. Neb. Ep. Sta. Quart. 4 (4): Kidson, E. 1931: Dry Years in New Zealand. N.Z.]. Sci. Tech. 13: Palmer, Robert S. 1958: Agricultural Drought in New England. N.H. agric. Ep. Sta. Tech. Bull. 97. P:crks, W. L.; Knetsch, J. L. 1959: Corn Yields as Influenced by Nitrogen Level and Drouth Intensity. Agron.]. 51: Penman, H. L. 1948: Natural Evaporation from Open Water, Bare Soil and Grass. Proc. roy. Soc. A. 193: Rickard, D. S. 1957: A Comparison Between Measured and Calculated Soil Moisture Deficit. N.Z.]. Sci. Tech. A38: Thornthwaite, C. W. 1948: An Approach Toward a Rational Classification of Climate. Geogr. Rev. 38: Thornthwaite, C. W.; Mather, J. R. 1955: The Water Balance. Dreel Inst. Tech., Publ. Climatol. 8 (1). Van Bavel, C. H. M. 1953: A Drought Criterion and Its Application in Evaluating Drought Incidence and Hazard. Agron.]. 45: Van Bavel, C. H. M.; Verlinden, F. J. 1956: Agricultural Drought in North Carolina. N.C. agric. Ep. Sta. Tech. Bull Van Bavel, C. H. M.; Carreker, John R. 1957: Agricultural Drought in Georgia. Ga. agric. Ep. Sta. Tech. Bull. n.s. 15.