**4. Results and discussion**

#### **4.1 Daily variation of temperature**

Figure 3 shows the variation of the tidal mud temperature at different depths from 1st April, 2006 to 8th April; 2006. It is seen that at 0.10 m and 0.20 m depth, the fluctuation of temperature was more prominent. However, from 0.50 m to 2.0 m depth, the diurnal variation was not so prominent. In the sub-surface region, the solar radiation affected the soil temperature more than the deeper part of the tidal mud.

This type of diurnal profile of temperature also agrees with the findings in Baeksu tidal flat in Korea (Yan-k et al. 2005). At 2.0 m depth, the temperature shows higher value than 1.0 m depth. This is probably due to the volumetric heat capacity of the tidal mud and the time lag for absorbing and releasing the heat during the summer and the winter. The peak temperature reached at different times at different depths. During the ebb tide, the time lag to reach the peak at different depths, is more than that at the high tide due to infiltration of sea water in the deeper depth.

Fig. 3. Variation of diurnal temperature with depth in the Higashiyoka tidal flat

Figure 4 illustrates one day (24h) variation of tidal flat mud temperature influenced by the solar radiation. It is seen that at 0.1 m depth, the peak value was reached when the solar radiation was also at the peak. At night, the temperature did not show any variation both during the ebb tide and the high tide time. This proves that the tidal mud temperature is only influenced by the solar radiation in the subsurface region. The tidal mud temperature at subsequent depths reaches the peak at different times, , with the time lag increasing with depth. The peak temperature was reached about 2:00 PM and the value was about 17 0 C at 0.10 m. The temperature at 0.50 m, 1.0 m and 2.0 m remained almost constant around 12-13 0 C. It is concluded from this Figure that time lag increased with increasing depth but the rate of increasing decreased with the increasing depth. Thermal properties of the tidal flat mud govern this type of phenomenon.

Fig. 4. Effects of solar radiation on the soil temperature in different depths

Fig. 5. Seasonal variation of temperature at Higashiyoka tidal flat

the rate of increasing decreased with the increasing depth. Thermal properties of the tidal

flat mud govern this type of phenomenon.

10 cm 20 cm 50 cm 100 cm 200 cm Solar heat (MJ/sq m)

**Soil Temperature (deg. cel)**

2006/4/5 0:16

2006/4/5 2:16

Depth (cm)

2006/4/5 4:16

2006/4/5 6:16

2006/4/5 8:16

Fig. 4. Effects of solar radiation on the soil temperature in different depths

2006/4/5 10:16

2006/4/5 12:16

0 5 10 15 20 25 30 35 40 Temperature (Deg. Cel)

2007.1.27 2007.2.28 2007.3.28 2007.4.28 2007.5.28 2007.7.28 2007.8.31 2007.9.30 2007.10.30

2007.11.28 2007.12.27

Fig. 5. Seasonal variation of temperature at Higashiyoka tidal flat

**Duration**

2006/4/5 14:16

2006/4/5 16:16

2006/4/5 18:16

2006/4/5 20:16

2006/4/5 22:16

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

**Solar Heat (MJ/sq.m)**

Fig. 6. Seasonal variation of temperature and consequently expansion and contraction tendency and acid treatment practice effects
