Moisture Web

10 Jan 06

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05 Dec 05

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21 Nov 05

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31 Oct 05

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10 Oct 05

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15 Aug 05

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25 Jul 05

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02 Jun 05

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13 May 05

10 January 2006

The rainfall of early December resulted in increased water contents in all paddocks, reaching a depth of at least 0.8 m in all but the native woodland, where it only reached 0.4 m, as can be seen in the comparison below of profiles on 1st and 15th December.

 

Little of this water had been lost from the two wheat paddocks when they were harvested in mid-December, because they had matured before the rain and provided an excellent mulch minimising evaporation from the soil surface.

Soil water in the annual pasture was topped up by the rainfall so that on 15 December it was almost as wet as it was at its wettest time in September. The bare surface encouraged more evaporation than from the wheat paddocks so that it was drier at 0.2 m. This further illustrates the limited ability of aging annual pasture to utilise available soil water.

The soil also wet up under the lucerne. Surprisingly, it also stayed reasonably wet for 2 to 3 weeks, with the pattern of strong water use at depth not resuming until towards the end of December (eg. see 1.0 m).

A similar response was seen in the native woodland, except that it did not wet as deeply, and it dried more quickly than the other paddocks. It is currently using water at all depths down to (and including) 1.2 m.

05 December 2005

After another couple of dry warm weeks (until the storms on 2 December which resulted in 27 mm at Coolamon Post Office), the effect of drying of the soil profile by the wheat crops and lucerne are even more pronounced (compare profiles from early December with that from 15 November).  They have now dried the soil to 200kPa (or close to it) down to 0.8 m, although the rain on 2 December caused some rewetting near the surface. With the exception of the grain only wheat at 1 m, the rate of soil water extraction has slowed down considerably since mid-November.

The contrast between the wheat and lucerne paddocks and the other two paddocks (annual pasture and native woodland remains as described on 21 November. The annual pasture has changed little since 15 November, except to wet up at 0.2 and 0.4 m in response to the rain on 2 December. The native woodland also wet up on 2 December at 0.2 m, but has started to draw down the soil water at 0.6 and 0.8m. Nevertheless, it is still much wetter than the wheat and lucerne paddocks, and continues to wet up at 1.4 and 1.6 m.

21 November 2005

After a pretty dry November to date, except for the very patchy storms on 7 November which seem to have bypassed the area (only 9 mm at Coolamon), the crops have continued to dry the soil profile. The wheat crops and the lucerne have dried the soil to the lower limit of the soil water sensor (200 kPa) at 0.2 and 0.4 m and are drawing water from deeper depths.

• Grazing wheat is showing strong soil water use down to 1 m, with some evidence of uptake at 1.2 m as well.

• Grain only wheat has used water strongly down to 0.8 m, and shows good evidence of use down to 1.2 m

• Lucerne is using water strongly down to 1.4 m, with evidence that it is also starting to be effective at 1.6 m

• The annual pasture continues to lag behind the crops and lucerne. It was the only site that showed evidence of the rainfall on 9 November, and remains much wetter than the wheat crops at 0.2 and 0.4 m, although it is drier than the native pasture at these depths. At all other depths it is the wettest of the 5 paddocks.

• The native woodland is the wettest site at 0.2 and 0.4 m, but shows evidence of water use at these depths. Although the soil is drying under the native woodland at 0.6, 0.8, 1.0 and 1.2 m, the shape of the curve suggests that this is the result of downward movement of water to deeper depths rather than water use by the vegetation. This is confirmed by the observation that the soil is still wetting at 1.4 and 1.6 m, the only site where there is strong evidence of this.

The results to date confirm that the annual pasture is shallow rooted, restricting its water uptake largely to the top half meter of the soil, and that the native woodland uses less water than agricultural crops in spring.

31 October 2005

Despite the continued rainfall (65 mm for October at the Coolamon Post Office), most paddocks have shown considerable evidence of soil water use during the last 3 weeks.

• All paddocks show strong soil water use at 0.2 m, with only the grazing wheat paddock showing the effect of rainfall (on October 20th and 30th).

• At 0.4 m, the four agricultural paddocks show strong soil water use (the strongest being the grazing wheat paddock); the native woodland shows very little water use.

• At 0.6 m, there is evidence of water use by the wheat and lucerne paddocks but not by the native woodland. Although the annual pasture paddock showed an apparent rapid drying around October 18, it has levelled out again, and this change, coupled with the similar rise in September, is attributed instead to the establishment and then draining of a temporary perched water table.

• At 0.8 m, it is not possible to be sure whether the drying in the wheat and lucerne paddocks is a result of plant water use or draining of water to deeper depths; because the change is gradual, the latter explanation is favoured at this stage. Neither the annual pasture nor native woodland sites show any substantial change at this depth.

Consistent with the evidence of water use above, the annual pasture and native woodland sites remain the wettest, and the two where the winter rainfall has penetrated deepest (now 1.6 m). As noted on 10 October, water use by native vegetation in winter and early spring is expected to be less than by crops and active pastures. The annual pasture did not put on serious growth until mid October, which explains why its water use lagged behind the other agricultural paddocks.

10 October 2005

After a long period characterised primarily by wetting, there are now signs of a strong drawdown of soil moisture in the top 0.4 m or so of the soil profile in most paddocks. At the same time, there is continued wetting at depth in most paddocks, as the water that entered the soil during winter continues to drain downwards.

The most pronounced drying has occurred in the two wheat paddocks an the perennial pasture, where at 0.4 m the soil is drier than 35 kPa. The least drying has occurred under the annual pasture, which has not developed much leaf area to date. This is also reflected by the fact that the annual pasture is the wettest of the paddocks at most depths, and was the first paddock to start wetting at most depths. The native woodland is the next wettest, and the only other paddock to have wet to 1.4 m so far this year (the wheat and perennial pasture paddocks are still wet at these depths from previous years). This is consistent with observations elsewhere that native vegetation often tends to conserve water through winter for use in summer.

The table below shows the progress of wetting in each of the paddocks; it indicates the date on which water reached each depth in each paddock. Dates highlighted in red are the earliest at each depth, while those in blue are the latest. From this it can be seen that annual pasture initially wetted up faster than other paddocks, was overtaken by the grain-only wheat paddock in July, but was again the first to wet at 1.4 m in September. This is consistent with the observation at the beginning of measurements that the annual pasture paddock was the wettest initially (closely followed by the cropped paddocks); it therefore required less water to fill each layer with the result that water travelled more quickly to depth. The perennial pasture paddock was the slowest to wet. This suggests it was the driest at all depths, so that more water was required to wet each depth and took longer to reach the depth below. This is also consistent with the observations before the rain commenced.

Date of arrival of winter rainfall at each depth in each paddock

	0.2 m	0.4 m	0.6 m	0.8 m	1.0 m	1.2 m	1.4 m	1.6 m
Grain-only wheat	16 Jun	20 Jun	28 Jun	9 Jul	26 Jul	20 Aug	-	-
Grazing wheat	14 Jun	19 Jun	01 Jul	16 Jul	28 Jul	24 Aug	-	-
Annual pasture	11 Jun	13 Jun	27 Jun	15 Jul	26 Jul	23 Aug	22 Sep	-
Perennial pasture	20 Jun	29 Jun	12 Jul	24 Jul	1 Sep	13 Sep	-	-
Native woodland	20 Jun	27 Jun	01 Jul	16 Jul	7 Aug	6 Sep	06 Oct	-

15 August 2005

Despite the 2 week break without much rain between 17 July and 3 August, the soil has remained wet at the shallower depths and continued to wet at the deeper depths. In the grain only wheat and annual pasture paddocks and the native woodland, wetting has occurred to 1 m, but only to 0.8 m in the grazing wheat and perennial pasture paddocks. These differences probably reflect the initial state of soil moisture in the different paddocks at the beginning of the season more than anything else.

25 July 2005

Once the winter rains started on 11 June, the soil moisture sensors in all the paddocks were quick to follow. The 0.2 m deep sensor in the annual pasture paddock was the first to wet up (on 11 June), followed within a few days by the other paddocks, the last being the grazing wheat paddock (16 June). After this, as the rain continued, the sensors at deeper depths gradually began to wet. By 24 July, water had penetrated to 0.8 m in all paddocks (although only just in the perennial pasture paddock). By this date the soil was also uniformly wet down to at least 0.6 m, as can be seen from the 24 July profile. Note that the two cropped paddocks and the annual pasture paddock are wetter at 0.2 and 0.4 m than the perennial pasture and native woodland. This is probably a consequence of (a) the drier subsoils, and (b) the more active water uptake in the perennial pasture paddock and the native woodland.

8 May 2005	01 June 2005